Search Engine

A search engine is a software system that finds, stores and ranks web content so people can get answers to their questions. Google, Bing and other engines crawl billions of pages, build an index of them, then use algorithms to show the most useful results for each search in a fraction of a second.

Written by Paul Hoda, SEO consultant since 2005. Last updated 8 November 2026.

How a search engine works

Most web search engines follow the same three stages. The details differ, but the basic process is well documented by Google and Microsoft.

  1. Crawling: automated programs called crawlers or spiders follow links and sitemaps to discover pages. Read more about crawling.
  2. Indexing: the engine processes each page, works out what it is about and stores it in a huge database called the index. This stage is called indexing.
  3. Ranking: when someone searches, the engine pulls matching pages from the index and orders them using many signals, often called ranking factors.

The results appear on a search engine results page, or SERP, which can include standard links, ads, maps, images, videos and AI-generated summaries.

For a fuller walk-through, see how search engines work.

The main search engines

Google handles the large majority of searches in the UK and worldwide, which is why most SEO work focuses on it. Other engines still matter for some audiences:

  • Bing: Microsoft’s engine, which also supplies results to other products, including Microsoft Copilot. See Bing SEO.
  • DuckDuckGo: a privacy-focused engine that draws many of its traditional results from Bing.
  • Yahoo: its web results have been powered by Bing for many years.
  • YouTube and Amazon: not general web search engines, but millions of people use them to search for videos and products.
  • Baidu, Yandex and Naver: leading engines in China, Russia and South Korea.

Search engines and AI

Search engines increasingly blend traditional results with generated answers. Google shows AI Overviews above many results, and AI assistants such as ChatGPT and Perplexity now run their own web searches before answering. These tools still rely on crawling and indexing content, so a site that is easy to crawl and clearly written is better placed to be found and cited.

Why search engines matter for SEO

Search engine optimisation is the practice of making a website easier for search engines to find, understand and trust. Because search engines send free, intent-driven visits, known as organic traffic, ranking well can be one of the most cost-effective sources of customers for a business.

Understanding how engines work also helps you avoid common problems: pages blocked from crawling, content that cannot be indexed, or pages that do not match what searchers want.

Common misconceptions

  • Search engines search the live web: they search their own index, a stored copy that may be days or weeks out of date for some pages.
  • You must submit your site to be found: engines discover most pages through links, although submitting a sitemap helps.
  • Paying for ads improves organic rankings: Google keeps paid and organic results separate.

Key Takeaways

  • A search engine discovers, stores and ranks web content to answer queries.
  • The three core stages are crawling, indexing and ranking.
  • Google dominates UK search, but Bing and others still send traffic.
  • Search engines search their own index, not the live web.
  • AI answers in search still depend on crawlable, indexable content.

Related Terms

Browse more terms in the SEO Encyclopedia.