Network-attached storage (NAS) is the name given to dedicated data storage technology that can be connected directly to a computer network to provide centralized data access and storage to heterogeneous network clients.
NAS differs from the traditional file serving and Direct Attached Storage in that the operating system and other software on the NAS unit provides only the functionality of data storage, data access and the management of these functionalities. Furthermore, the NAS unit does not limit clients to only one file transfer protocol.
NAS systems usually contain one or more hard disks, often arranged into logical, redundant storage containers or RAID arrays, as do traditional file servers. NAS removes the responsibility of file serving from other servers on the network and can be deployed via commercial embedded units or via standard computers running NAS software.
NAS uses file-based protocols such as NFS (popular on UNIX systems) or Common Internet File System (CIFS) (used with MS Windows systems). Contrast NAS's file-based approach and use of well-understood protocols with storage area network (SAN) which uses a block-based approach and generally runs over proprietary protocols.
Minimal-functionality or stripped-down operating systems are used on NAS computers or devices which run the protocols and file applications that provide the NAS functionality. A "leaned-out" FreeBSD is used in FreeNAS, for example, which is open source NAS software meant to be deployed on standard computer hardware. Commercial embedded devices and consumer "network appliances" may use closed source operating systems and protocol implementations.
The Wikipedia entry for the term NAS can be found here
In the video below, Darren from Hak5 builds a Network Attached Storage server for their home network using a spare PC, a compact flash card, and some BSD. Simple but sweet.