NAS - Network Attached Storage Appliances

What's NAS?
A network-attached storage (NAS) device is a server that is dedicated to nothing more than file sharing.
NAS does not provide any of the activities that a server in a server-centric system typically provides, such as e-mail, authentication or file management.
NAS allows more hard disk storage space to be added to a network that already utilizes servers without shutting them down for maintenance and upgrades.
With a NAS device, storage is not an integral part of the server. Instead, in this storage-centric design, the server still handles all of the processing of data but a NAS device delivers the data to the user.
A NAS device does not need to be located within the server but can exist anywhere in a LAN and can be made up of multiple networked NAS devices. 

Why NAS?
Network-attached storage (NAS) is hard disk storage that is set up with its own network address rather than being attached to the department computer that is serving applications to a network's workstation users. By removing storage access and its management from the department server, both application programming and files can be served faster because they are not competing for the same processor resources. The network-attached storage device is attached to a local area network (typically, an Ethernet network) and assigned an IP address. File requests are mapped by the main server to the NAS file server.
Network-attached storage consists of hard disk storage, including multi-disk RAID systems, and software for configuring and mapping file locations to the network-attached device. Network-attached storage can be a step toward and included as part of a more sophisticated storage system known as a storage area network (SAN).
NAS software can usually handle a number of network protocols, including Microsoft's Internetwork Packet Exchange and NetBEUI, Novell's Netware Internetwork Packet Exchange, and Sun Microsystems' Network File System. Configuration, including the setting of user access priorities, is usually possible using a Web browser. 

Differentiating between NAS, DAS and SAN
A NAS Appliance does just one thing - providing vast amounts of storage accessible from your network at the lowest TCO (total cost of ownership).
NAS is distinguished from DAS (Directly Attached Storage) in that the storage is accessed on another "box" through the network, instead of via direct SATA or SCSI cabling.
SAN means Storage Area Network, and is the overall space in which servers and workstations are able to access storage across the network from NAS appliances.
NAS has many advantages over DAS, such as its modularity - you can keep adding NAS appliances to your network almost indefinitely without having to add new servers, whereas you can practically only add a small number of DAS boxes to a single server.