You've probably used search engines like Google, HotBot or AltaVista.
The thing we call a search engine is actually a set of programs. One program, called a robot, periodically moves through the Web following links and capturing information about Web pages. Another part of the search engine builds indexes from the information found by the robot.
Search engines allow users to search, display and organize information found in the index. Some options and interfaces vary between search engines but most of the basic ideas for searching them are the same.