NAS stands for Network Attached Storage, a mechanism for providing file-level access to storage across a network.
NAS systems are generally formed of a RAID array (or multiples thereof) of disks, and a file server providing a number of different protocols, often NFS and CIFS (SMB), and sometimes AFP which provide file-level access to a number of different clients.
NAS devices can be compared to SAN systems, where the NAS provides storage and a filesystem, but the SAN only provides storage, and leaves the filesystem level decisions down to the client.