Network attached storage (NAS) is file-level computer data storage connected to a computer network. NAS provides both storage and a file system.
In the basic sense, a NAS unit is a computer connected to a network that only provides file-based data storage services to other devices on the network. Although it may technically be possible to run other software on a NAS unit, it is not designed to be a general purpose server. For a more in-depth technical description, search for this definition below.
FYI: NAS is not to be confused with SAN (Storage Area Network), which provides only block-based storage and leaves file system concerns on the client-side. Despite their differences, SAN and NAS are not mutually exclusive, and may be combined as a SAN-NAS hybrid, offering both file-level protocols (NAS) and block-level protocols (SAN) from the same system. An example of this is Openfiler, a free software product running on Linux.