As the name implies, Web search engines find Web sites containing the words you are looking for. Unlike article databases or library catalogs, search engines usually rely on automated means, typically called spiders, robots, worms or ants, to identify the pages they will include.
Search engines are not always very "bright," they search strictly for the "strings" of characters you enter. The words you enter may actually be a string of characters "buried" in another word. A search for "sand," for example, may turn up matches with ampersand, sandwich or sandstone.