A program that "crawls" across the World Wide Web, automatically collecting Web pages. Most spiders will follow every link on a page, cataloging each page, until it comes to a dead end. Then it will start over on a new page. Spiders are used primarily by Web search engines to gather data for the search engine's database. Search engines don't actually search the entire Internet when a user enters a search term. Instead, they look at the database of Web pages collected by their spider.