From LevelTenWiki

A search engine allows people to find web pages/sites that contain a given keyword or phrase. When a user enters and makes a query the engine returns the pages/sites that it determines to best match the search criteria. Each search engine has its own method of indexing a page/site to determine which ones rank best. Note that most public search engines, i.e. Yahoo, AltaVista, Lycos, actually use both a page search engine and a directory.

Search Engine Types

• Search engine - Indexes the content of web pages automatically using existing hyperlinks and other meta data
• Search directory - Indexes text descriptions for a site

Search Engine Scope

• External search engine - Used to search pages/sites accross multiple domains, i.e. Yahoo, AltaVista
• Internal search engine - Used to search pages within a web site.

Examples

The following is a list of popular search engines. Often these search engines are only portals to someone else's database. For example, Yahoo maintains its own directory for web site results yet uses Google for its web pages results. Yahoo used to use Inktomi for its web page results, which is used by Hotbot and others. The list is just a list of a few popular search engine, their are literally thousands of search engines and directories on the Internet.

• Google - Search engine
• MSN - Search engine and directory
• AltaVista - Directory (by Looksmart) and search engine
• Yahoo - Directory and search engine (by Google)
• Ask - Search engine
• Hotbot - Directory (by Open Directory) and search engine (by Inktomi)
• InfoSeek/Go - Search engine
• Lycos - Directory (by Open Directory) and search engine (by Fast & Direct Hit)
• Open Directory - Directory