Search engines go to a page, look at the contents, condense the information and store it in a (huge) database. Then they follow all the links on that page, and do store the information that they find over there, etc. They "crawl" like a spider over the web for 24 hours a day! The major engines are displayed in bold on the top of this page.

When you type a search word in one of those Websites, the software will look in their database and make a list of all the pages they have with those particular words in it. This could be 300.000 sites, but the one on top of your list has the words you were looking for most frequently and in the most appropriate places. (title, summary, pictures, etc.)

However, if your search finds 300.000 sites, your search might be too broad. You need a few tricks to get to where you want. See: search engine search strings.