4 What's going on

4.1 What is a search engine?

Search sites are powered by a search engine: a program that can search for web pages that match your query, and then return a list of hits. The list of hits arrives at your browser as a web page, complete with links to the pages the search engine has found. A website such as the Open University may provide its own search engine which searches only its own web pages, but the big search sites claim to search the entire Web. This is a stunning claim: Google claims to search over four billion web pages yet usually finds what I want in a fraction of a second. How is it done?

A schematic diagram showing a search engine answering a query. A computer runs a web browser and the query ‘GM food’ is typed into a search box. An arrow shows that when the Search button is pressed the query is sent to the search engine – which looks like a railway engine puffing steam! The search engine draws on the index (shown as a stack of pages) and sends results back to the browser

Last modified: Thursday, 2 August 2012, 12:30 PM