It's important to know the difference between search engines and directories, they are often confused.
Search Engines: Also called "spiders" or "crawlers," search engines constantly visit web sites on the Internet in order to create catalogs of web pages. Because they run automatically and index so many web pages, search engines may often find information not listed in directories.
Directories: Unlike search engines, directories are created by humans. Sites must be submitted, then they are assigned to an appropriate category or categories. Because of the human role, directories can often provide better results than search engines. Yahoo is an example of a directory.
Hybrid Search Engines: To further confuse matters, some search engines also have an associated directory. These are sites that have been reviewed or rated. For the most part, these reviewed sites do not appear as the "default" when a query is made to a hybrid search engine.