How search engines choose and rank results
Computer Science · Key Stage 2 · The School
Crawling and indexing
A search engine does not search the web when you type. Long beforehand, programs called CRAWLERS follow links from page to page, and what they find is stored in an INDEX — a giant list of which words appear on which pages. Your search is matched against that index, which is why the answer comes back in a fraction of a second.
Why this one is first
Ranking uses many signals at once: how well the page matches your words, how many other pages link to it and how trusted those are, whether it loads well on your device, what language and country you are in, and what people who searched something similar went on to click. Results are also personalised, so two people searching the same words may see different lists.
Searching on purpose
Small changes make a large difference. Put a phrase in "quotation marks" to demand those exact words in that order. Add site: to search within one site. Add a minus sign before a word to exclude it. Ask a fuller question rather than one vague word. Most poor searches are not the engine's fault — they simply did not say enough.
Page A is written by a scientist, has no links pointing to it, loads slowly, and was published in 2009. Page B is written by a marketing team, is linked from a thousand sites, loads fast, and was updated last week. B will very likely rank higher. Nothing in that list is a measure of truth — it is popularity, speed and freshness. Which is why the top result is a place to start, not a verdict.