In computing, a directory, catalog, or folder, is a particular area of the file system used for organizing files. The name folder is especially used on some operating systems, such as Mac OS and, increasingly, Microsoft Windows.
Note that the folder metaphor may be misleading with regard to things like file permissions on UNIX: To rename or delete a file you need write permission to the directory that contains the file. This is perfectly understandable if the directory is seen as a list of filenames but not if it is seen as a container (as folder implies).
In graphical user interface (GUI) environments, folders are often depicted with icons which resemble physical file folders such as those of a file cabinet in an office.
If you imagine the computer's file system as a file cabinet, high-level directories may be represented by the drawers, and low-level directories or "sub-directories" may be represented as file folders within the drawers. Operating systems like Microsoft Windows, MS-DOS, and UNIX support practically limitless levels of sub-directories (or folders within folders.)
The word directory is used in computing with a different sense: a central repository of information related to management of a computer or a network of computers. This includes data on users, applications, hosts, network devices, security credentials and more. The directory, as opposed to a database, is heavily optimized for reading, with the assumption that data updates are very rare compared to data reads.
The Distributed Management Task Force (DMTF) is creating standards related to the information stored in such directories, and the protocols and APIs used to access it. The main product of these efforts is a common information model (CIM) for management.