Web search engines keep track of Web pages and other forms of information on the Internet — including PDF files, PowerPoint files, Word files, blogs, and newsgroup posts — by locating documents on Web sites and entering them in a searchable database.

Leading Web search engines include

Keep two cautions in mind as you use Web search engines. First, the sheer size and rapid growth of the Web make it difficult for search engines to create a complete index (recent estimates suggest that at least one trillion pages have been created). As a result, you should use more than one search engine. Even if you don’t find what you’re looking for in your first choice, you might find it in another. Second, Web pages can be moved, deleted, or revised. This means that a search engine’s results can be inaccurate.