How Do Search Engines Work?
Search engines compile their databases with the aid of spiders (a.k.a. robots). These search engine spiders crawl the Internet from link to link, identifying Web pages. Once search engine spiders find a Web site, they index the content on those pages, making the URLs available to Internet users. In turn, owners of Web sites submit their URLs to search engines for crawling and, ultimately, inclusion in their databases. This is known as search engine submission.

When you use search engines to find something on the Internet, you're basically asking the search engine to scan its database and match your keywords and phrases with the content of the URLs they have on file at that time. Spiders regularly return to the URLs they index to look for changes. When changes occur, the index is updated to reflect the new information.