Storage area network
To understand the difference between a SAN and Network Attached Storage (NAS), the difference between the file system and storage must be made clear.
The file system is the interface that allows the user to access their data that is stored in physical storage (such as hard disks). From the point of view of the user, their data is saved as files. The file system takes care of storing the file data, which might actually be broken up into pieces and stored in different locations in storage.
On a personal computer, the file system and storage are on the same machine (Direct Attached Storage). In Network Attached Storage, the file system and storage are on a remote server. In a Storage Area Network, the file system is on the local machine, and the storage is on a remote machine.