Directories are a subject guide, typically organized by major topics and sub-topics. The best-known directory is the one at Yahoo!. The biggest directory is the Open Directory Project, which has members of the public editing the categories. Many other sites now use directories like Yahoo's including major search engines. Such as MSN ,Google and so on.
Directories use human editors to review most sites' subject matter and quality, and for this reason are considered authorities by other search engines which use directory listings to categorise their own indices .