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How a search engine finds an answer among billions of pages in a fraction of a second.

3 min read Last updated 3 Nov 2026
A magnifying glass over a list of web page cards, with the top one starred.

The short version

A search engine does its work in advance. Programs called crawlers follow links around the web and copy what they find. The engine then builds an index, like the one at the back of a book, listing which pages contain which words. When you search, it looks in that index, ranks the matches, and shows the best.

I always assumed a search engine searched the web when I pressed the button.

It searches its own copy, made earlier.

That is why it is quick.

Step one: crawling

Crawlers, sometimes called spiders, are programs that visit a page, copy information from it and follow links to other pages, then repeat the process. They are always at work, revisiting pages to see what has changed. Pages that no other page links to are hard for them to find.

Step two: indexing

The engine analyses each page and builds an index. Like the index in a book, it records which pages contain particular words and topics. Looking up "kettle" gives a list of pages at once, without reading any of them again. The index also records where on the page the word is, and what the page is about.

Step three: ranking

Thousands or millions of pages may match a search. The engine must decide the order. It looks at many clues: whether the words are in the title and headings, whether it is a recent and fast page, where you are, what language you use, and how many other trusted sites link to it. How search ranks results looks at this more closely.

Hundreds of billions

The number of pages in the index of the largest search engine, according to its maker. The answer comes back in a fraction of a second.

The top result is the most true.

The top result is the one the engine rates as most relevant and most trusted by its measures. That is often useful, but it is not a check of accuracy, and adverts can appear above or among the results, usually marked.

Different for different people

Two people searching the same words can see different results. The engine takes account of language, place and sometimes past searches. A search for "pizza" shows pizza places near you, not near someone else.

Note: engines differ in how they work and how they rank. This describes the general approach.

What search cannot see

Some pages are never indexed. Pages behind a login, such as your email or a private account, are not open to crawlers. Site owners can also ask crawlers to stay away. So a search shows a large part of the web, not all of it.

Search engines earn most of their money from adverts shown next to results, which are paid for and normally marked. The unpaid results are listed by the engine's own judgement.

Key takeaways

  • Crawlers follow links and copy pages.
  • An index lists which pages contain which words.
  • Ranking decides the order of the matches.
  • You search the engine's copy, not the live web.
  • Results can differ from person to person.

Now you know.

Search is done in advance.

An index is a list of words.

The order is a judgement.

Last reviewed 3 Nov 2026

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