NAS is a term commonly used for what most people think of as a “file server”; a device that has the specific purpose of exposing a filesystem to clients over a network. The network technology most commonly used is Ethernet, although some niche products also support other network types. The main difference between a NAS and a SAN (Storage Area Network) is that a NAS serves complete files using a network filesystem such as SMB, CIFS or NFS; a SAN presents block storage, requiring client machines to add their own filesystem.
Contrary to a traditional file server, NAS appliances are built specifically for the single purpose of serving files. Most have advanced features such as asynchronous and/or synchronous replication to other appliances for disaster recovery or backups, fine-grained access control and snapshot support.
Smaller NASes use internal, direct-attached storage; larger models can be expanded by adding direct-attached disk trays, by clustering several appliances together, or by tapping into a Storage Area Network.
NetApp FAS6040
Larger NAS appliances use custom-built hardware, but smaller models are generally built on top of general purpose operating systems such as Linux running on top of off-the-shelf hardware. Technically speaking, some of these might fall under the “file server” moniker; but since the operating system has been trimmed down to the bare essentials the term NAS is often used for these anyway. Good examples are FreeNAS (running on Linux) and Sun’s Open Storage.
On the other end of the spectrum, the picture is also starting to get a bit fuzzy; there are several NAS vendors that are starting to add support for block-based access protocols such as iSCSI and FCoE. This makes their products fall in both the NAS and SAN categories.
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