A directory or folder is an entity in a file system, which contains a group of files and/or other directories or folders. A typical file system may contain thousands (or even hundreds of thousands) of directories or folders. Files are kept organized by storing related files in the same directory or folder. A directory contained inside another directory is called a subdirectory of that directory. Together, the directories form a hierarchy or tree structure.
A directory is an online search service. There are general directories (e.g., Yahoo!), commercial directories, which are the equivalent of paper-based industry guides, and specialized directories, which are topic-oriented and may provide consumers with technical and evaluative information about goods or services, in addition to simple search support. In general, a directory's sole source of income is advertising.