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?