A network-attached storage (NAS) device is a server that is dedicated to nothing more than file sharing. Network-attached storage does not provide any of the activities that a server in a server-centric system typically provides, such as email, authentication or file management.
NAS is a Storage-Centric Design
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.