A program that automatically fetches Web pages. Spiders are used to feed pages to search engines. It’s called a spider because it crawls over the Web. Another term for these programs is webcrawler.
Because most Web pages contain links to other pages, a spider can start almost anywhere. As soon as it sees a link to another page, it goes off and fetches it. Large search engines, like Alta Vista, have many spiders working in parallel.
Also see How Web Search Engines Work in the Did You Know . . . ? section of Webopedia.