Network Attached Storage
Network Attached Storage, often shortened to NAS, as implied by the name is a form of file storage made available to the network. An early example of such a system would be the Unix-World's own NFS service, where a file system store is available from across a network.
Typically a NAS system has a role of a server though it does not always take the form of a typical server, such as those that are usually rack-mounted and made by HP or Dell. Some come in a small, near portable form factor, with an embedded operating system. All that is needed for these devices to function is a power supply and a network lead. These usually have a web-based configuration utility similar to those found in home broadband routers. Obviously a real server affords the luxury of RAID-enabled storage and multiple network cards and a "full-featured" operating system.
Dependant on the intended audience for a NAS server will dictate the types of protocols it will serve. The majority of portable NAS servers support CIFS (formerly SMB by Microsoft, known as Samba on Open Source operating systems) since the market share will already have Windows-based operating systems that natively support it. Other, more advanced, NAS servers have the capability to provide protocols for NFS, FTP, WEBDAV, AFP, iSCSI, UPnP as alternatives to CIFS alone.
FreeBSD can support NFS and FTP natively and CIFS through the open source implementation called Samba as well as initial support for iSCSI (as a "target" in iSCSI terminology) and other 'www'-borne protocols.
There is an active project called FreeNAS (official homepage)from which the name takes two meanings. The obvious is that it is a free implementation of a NAS server. The other is that it refers to the fact that it is based on FreeBSD. It is technically based on the work of the FreeBSD-derived firewall project called m0n0wall (note the use of zeros in the name, official homepage), a stripped down FreeBSD operating system for embedded, dedicated-use projects such as firewalling and NAS serving. FreeNAS can take an old or low specification computer that is no longer used and, when fitted with a modest sized hard drive, turn it into a fully functional NAS server supporting CIFS, NFS, FTP, AFP, iSCSI and UPnP, backup systems RSYNC and Unison and Dynamic DNS functionality.