How Search Engines Work

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Search engines visit billions of pages on the World Wide Web using web crawlers (also known as spiders or bots) following links from page to page. The discovered pages are then added to an index that search engines pull results from for particular search queries.

In a few words the process that enables users to search and find relevant web pages using a search engine can be described as follow: A Search Engine uses bots to create a database of web content called an index, which is then employed by the search engine algorithm to retrieve the most relevant information from in response to a user search query.

  1. Search Engine Bot: A web crawler following links on already known pages to discover new pages on the web
  2. Search Engine Index: A digital library storing information about webpages
  3. Search Engine Algorithm: Computer program tasked with matching results from the search index with search queries
  4. User Search Query: A user input (text, image or voice) instructing the search engine about the subject of their search

Although every search engine aims to provide the most relevant search results to its users, it is important to outline that they do not make any profit in doing so. Money-wise, the search engines rely entirely on paid search results that go alongside the organic search results.

Making the distinction between the two is of paramount importance. This description of how search engines work including the said search engine bots, the index and the algorithm as well as SEO in general applies only to organic search and not at all to paid search.

Each search engine has its own process for building a search index and is maintaining it independently. As of 2025 Google’s market share in this space is 93% in the UK and 87% in the US.

Why is it Important to Know How Search Engines Work?

  1. Understanding the basics of how search engines work is imperative for Onpage SEO. It ultimately helps with the answers to the most rudimentary questions as to why certain pages rank and others do not. The more thorough your understanding of the elements and processes behind search engines becomes, the more equipped you are to understand the full picture of what it will take to rank for your target keyword.
  2. Understanding some basic aspects of how search engines work with particular reference to how they interpret hyperlinks between websites to estimate authority, relevance, trust of any website on the web will also serve as a foundation for understanding Link-Building and its importance for further amplifying your Onpage SEO efforts.
  3. A beyond-basic understanding of how search engines work will also enable you to delve into the pivotal technical SEO factors that directly feed into how effective your Onpage Optimisation efforts will be at achieving your business objectives. It is through technical SEO that you will be able to further refine how much of an impact your Content Strategy and its overlayed Onpage Optimisation will have.

Deconstructing the Search Engine Process

At its core, the behind-the-scenes process that allows Search Engines to return a collection of website links in response to user search queries includes 4 steps, starting with data-collection in the form of Crawling and Rendering, then storing in the form of Indexing and finally, organisation in the form of Ranking, as described below:

  1. Crawling: The data-collection process of search engines through the use of bots (i.e. Googlebot) to discover new and updated content on the web.
  2. Rendering: The process search engines employ to execute a webpage’s code and generate the visual, interactive version of a web page that a human user would see.
  3. Indexing: The process of storing internal notes on the previously Crawled and Rendered web pages in a proprietary database, so that the web pages in question can be retrieved promptly in response to user search queries relating to the page’s content.
  4. Ranking: The process employed to determine the order of search engione results in response to user search queries, governed entirely by the Search Engine’s proprietary algorythm.

1. Search Engines Bots Discover Information by Crawling Website Pages

Frankly, to actuate technical SEO to your benefit on any given website you will need a thorough understanding of how search engines work beginning with the information discovery process of the search engine bots, often referred to as Crawling. The discovery process begins with a known list of URLs. Search engines can discover these in several ways:

From backlinks: Google has an index of over 400 billion webpages. When someone links to a new page from a known page, search engines can find it through the hyperlinks on these pages.

From sitemaps: Sitemaps tell search engines which web pages and files website owners want to be both crawled and indexed. This is another method that enables search engines to discover URLs.

From URL submissions: Search Engines allow site owners to request crawling of individual URLs from within their proprietary tools (i.e., Google Search Console for Google Search).

The role of robots.txt & robots meta tags: While sitemaps instruct robots on what pages are flagged for indexing, the role of the robots.txt file and robots meta tags is exactly the opposite. Both are employed to restrict robots from crawling or indexing certain pages, but it is also the place that specifies the URL of your XML sitemap.

If a crawler or search engine bot cannot access or efficiently navigate your website, your pages simply won’t be discovered and subsequently won’t appear in search results, regardless of their quality. Optimizing your site’s crawlability through proper site structure, internal linking, sitemaps, and managing crawl budget is crucial for ensuring search engines effectively discover all your valuable pages and content within it.

2. Search Engines use Rendering to interpret pages the way Website Users are intended to

Rendering is when search engines attempt to run a page’s code in order to extract key information from the crawled pages and perceive it as a user would. Understanding Rendering will enable you to ensure all the content that is part of any given page makes it into the search engine index in a predictable way and not only a part of it. Without this understanding parts of your onpage content might remain invisible to the systems meant to find it.

Learning how to ensure all your page content is accessible to search engine bots and fully rendered by them is primarily important because if only a part of the page content is accessed, it puts you at a disadvantage as the unrendered content, potentially including important keywords and internal links, won’t be considered when establishing your Page Ranking for various keywords.

Once crawled and rendered any given page is ready for Indexing, but an important sidenote on Rendering would be that even if any given page has been indexed – it does not by extension imply that all of its content made it into the index. So you might remain unaware that not all the page content is employed in building up your website or page rankings. This is, of course, unless you understand and test the rendering of the page.

3. Indexing defines what ends up in Search Engine databases

Indexing is the process of adding information from crawled pages to a search index. The search index is what one searches when using a search engine. That’s why getting indexed in major search engines such as Google is so important for businesses. Users can’t find your businesses unless they’re in the index.

Crawling and Rendering together with the indexing rules specified on any given page will enable either the inclusion or exclusion of a page from the search engine index. If your pages are not successfully indexed, they will not get the chance to rank in search results, even if crawled and rendered perfectly.

At its core, it’s important to understand indexing in order to be able to make the distinction between an indexable and non-indexable website page, one that has been indeed indexed and one that hasn’t and if a page is indexable but not indexed – understand the root causes that prevents the indexing of the page by any given search engine.

4. Ranking is Search Engine Algorithm’s matching of Search Queries to a prioritised list of website pages from the Search Engine Index

Google also needs a way to shortlist and prioritize the 400 billion landing pages for all keywords from billions of its users worldwide to a more manageable number. This is where search engine algorithms come into play. Their sole purpose is providing the user a list of search results in the order it estimates to be most likely to solve the user’s search query.

Search engine algorithms are formulas that match the user’s keyword to relevant landing pages stored in the index in the form of search results. The search results are prioritized according to said search engine algorithm, with the result at the top being considered the single most relevant and useful for any one keyword in question.

No person knows every search engine ranking factor, not only because they vary across search engines but also because the search engines do not publicly disclose them. Nonetheless, search engines, including Google, have, in fact, disclosed to the public some of the key ranking factors as well as some best practices for SEO, which can prove quite useful to those with or without an SEO background.

Some of the key ranking factors are backlinks, content relevance and freshness, page loading speeds, and mobile-friendliness. It’s worth noting that users may see different results for the same keyword depending on their location, the language set on their browsers, and their search history.

Understanding the nuances of how search engines, primarily Google, rank pages for keywords and use them in its AI overview is what allows you to increase the visibility in organic search and by extension drive organic traffic through SEO. Understanding the multitude of factors that influence how a search engine orders results and the intricacies of each factor in isolation as well as in relation to other factors enables you to make informed judgment calls on various aspects of the website as a whole as well as individual landing pages in order to uplift them for target search queries and user intents in the SERPs.

Ultimately, the understanding of the search engine ranking factors, each contributing to the ordering of search results by the search engines is what allows the Search Engine Optimisation to take place. Mastering the principles of ranking allows you to position your content favorably against competitors and effectively connect with your target audience.

1990

The World Wide Web, First Website and First Search Engine

The World Wide Web, the first website and the first search engine have all appeared within the timespan of a year. Incidentally, the first search engine, a small project by a McGill University student in Montreal, was the one to precede the other two. These three concepts were tied together from the very start and have been heavily reliant on each other ever since.

The First Few Thousand Websites

The first websites for general public use began to emerge between 1993–94.

1994

Thousands of Search Engines Failed

In the early days of the World Wide Web, the internet was ambushed by a mountain of search engines, each with its own cataloguing techniques, search algorithms, target audiences and all kinds of special features. Two decades later, it is now safe to say that all, but one of them, have failed.

Understanding the World’s Knowledge
The vast majority of early Search Engines had fairly narrow targeting, which meant you had to use different search engines for different things. All of them relied heavily on Webmasters to understand what the content was about which raised a whole set of issues, but most importantly SPAM.

2. Match it to Users Needs
All Search Engines were a mixture of Search Engines, Directories, News Sites, and Catalogues, among other things, tampering with users’ needs at every interaction. In other words, the Search Engines grew better at cataloguing the world’s knowledge, but it happened at the expense of matching it to users’ needs.

3. Instantly!
The special features and excessive advertising raised the search loading speeds. Most search engines hosted an index of websites and webpages that would sometimes take weeks or months to update. In effect, search results took a long time to load and once the search results appeared some of the content was no longer available or modified beyond the point of relevance.

The First Few Million Websites

The World Wide Web hit 1 million websites at some point in 1997. As you might expect, getting any kind of additional interesting facts about those times becomes increasingly more difficult.

1998

One Search Engine Succeeded

It wasn’t until the publication of an academic paper called ‘The Anatomy of a Largescale Hypertextual Web Search Engine’ by the muscovite Sergey Brin in 1998 which grew into today’s Google, that the idea of search engine really took flight. At first, Google was hardly any better than the other search engines, it was mediocre at understanding the content it was indexing and it wasn’t a leader in matching it to user needs, either. However, it has spread its dominance over the search engine game almost overnight, which is most often attributed to its slick user interface that had virtually no features. 

In the long haul, what allowed the equation of Google to Web Search in the minds of billions was exactly what all the other search engines failed at – that initial vision of:
• understanding the world’s knowledge 
• matching it to users’ needs
• seamlessly

The crucial factor has been their ability to understand information in increasing depth, with limited reliance on Webmasters and match it to user needs in a manner that is universal and rich and with increasing accuracy over time, through what we most commonly know today as algorithm updates to their search engine.

They delivered on the instancy aspect too, by making today’s search results virtually real-time. In simple terms, as a result, Google has managed to accomplish what none of the other search engines did: Return a fairly relevant collection of links for almost any search query, instantly.

The First Billion Users

It might come as a shock, but it took search engines less than 10 years to get to a billion users.

2005

The Relationship Between the Search Engine and SEO

As search engines turned mainstream, manipulating them for commercial gain has become the informal definition of SEO and its outcomes widely considered to be Web-SPAM. By the year 2005, Google started to take Web-SPAM very seriously and hasn’t stopped working on it ever since.

At the same time, they empowered webmasters with tools to drive commercial value through relevance to their customers as opposed to unethical practices.
• Launched Google Analytics and Google Search Console that allowed website owners to better understand the behaviour of their customers online
• Started supporting initiatives on content mark-up that aided search engines to better understand the knowledge behind the information:
▫ Sitemaps and robots.txt files to give webmasters control of what gets indexed
▫ Canonical tags and pagination to fight duplicate content and aid content attribution
▫ Structured Data (schema.org vocabulary) to aid the understanding of semantics
• Put an emphasis on branding to increase the websites’ commitment to their online presence
▫ Started taking reviews into account
▫ Started using signals from social media
• Made the web relevant to our location and our devices
• Punished unethical SEO practices like
▫ Keyword-stuffing and over-optimisation
▫ Link-farms and paid links
▫ Duplicate content and thin content

The First Trillion Pages

At some point before 2010, Google has revealed that their index at the time was already storing some 1 trillion pages.

2010

Intent-Based Search and Rich Search Results

Although Google’s war on SPAM has continued, it also started stretching the idea of what search can accomplish, putting users’ search intent at the centre. By increasing its:
• Understanding of semantics and ability to recognise context in order to
• Measure user behaviour, Google has learned to understand and anticipate user intent on an individual basis and thus increase the accuracy with which they can match content to search queries.

In simple terms, returning a fairly relevant collection of links for almost any search query has, as a result, been replaced by returning the web’s single most relevant resource, which is able to fully solve the search query.

Some of the newly developed technologies:
• Autosuggest
• SERP-features and site-links
• Mobile and local
• Encryption and security, 
• Machine learning (RankBrain)
• Policy on interstitials

The First Trillion Yearly Google Searches

At a point before 2015, Google has revealed that they get at least a trillion searches per year, a number that has been growing ever since.

2015

The Advent of Real-Time

In 2015, Google announced that you’re able to get almost any data on what’s happening in real time.

Half of the World Population on Search

By 2016, according to some sources, the World Wide Web had 1 billion websites and over half the world’s population using search engines. However, as it happens, more than 80% of these websites are inactive.

2020

BERT Language Interpretation

Although BERT was introduced before 2020, it was rolled out over a number of consecutive years. Google claims that BERT has helped the search engine “understand searches better than ever before”.

Any efforts directed at a website in the public domain that are not naturally geared towards fulfilling these underlying principles, will become a SEO liability in the long run.

Sergiu George