Network-attached storage (NAS) is a hard disk storage system  it includes RAID Configuration which is helpful for fault tolerance and gives maximum protection and security of data, NAS has its own LAN IP address rather than being attached to the department computer that is serving applications to a network's workstation users. It has it's own software for configuring and mapping file locations to the network attached devices. A NAS can be included as part of a more comprehensive solution like SAN (Storage Area Network)
NAS and SAN are different things, we can say NAS can be a part of SAN. 

Commercial NAS:

There are many Expensive NAS Systems e.g; WD, Iomega, LG, NetGear, QNAP, etc etc.
I will not recommend to go for the above expensive systems, I have used some of these but the problem in these systems is that there is only Hard Disk which is main hardware and the machine is having poor performance some are using USB Connections for Hard disks, so I suggust you can create your own NAS instead of buying above expensive systems for this you can use your computer with  some hard disks and OpenSource Operating Systems e.g.

This Blog will explain you how FreeNas will be Installed and Configured 

FreeNAS - Your Network Storage

FreeNAS is an embedded open source NAS  based on freeBSD, it supports popular protocols like CIFS (samba), FTP, NFS, TFTP, AFP, RSYNC, Unison, iSCSI (initiator and target) and UPnP.
It supports Software RAID (0,1,5), ZFS, disk encryption, S.M.A.R.T/email monitoring with a WEB configuration interface FreeNAS can be installed on Compact Flash/USB key, hard drive or booted from LiveCD.

FreeNas Video Tutorial

This is complete Training Video of FreeNas

Openfiler Installation Video

This is complete Training Video of Openfiler


System Requirements : 
- System with CD ROM
- 512MB Ram or More
- 256 MB Compact Flash or Flash Disk
- IDE or SATA Hard Disk