Storage Area Network is a special type of network that connects computers and disk devices; in the same way as SCSI cables connect disk devices to one computer.
Any computer connected to SAN can send disk commands to any disk device connected to the same SAN. On the physical level, SAN can be implemented using FDDI, Ethernet, or other types of networks.
Some disk drives or arrays have "dual-channel" SCSI controllers and can be connected to two computers using regular SCSI cables. Since both computers can send disk read/write commands to that shared disk, this configuration has the same functionality as a one-disk SAN.
SAN provides Shared Disks, but SAN itself does not provide a Shared File System. If you have several computers that have access to a Shared Disk (via SAN or dual-channel SCSI), and try to use that disk with a regular File System, the disk logical structure will be damaged very quickly.
There are two main problems with Shared Disks and regular File Systems:
Disk Space Allocation Inconsistency:
If computer X and computer Y both connected ("mounted") a shared disk, their File Systems loaded the File Allocation Tables into each computer's memory. Now, if some program running on computer X tried to write a new block to some file, the File System running on that computer will check its File Allocation Table and free blocks list, and it will allocate a new file block number 13477 to that file.
The File System running on that computer will modify its File Allocation Table, but it will have no effect on the File Allocation Tables loaded on other computers. If an application running on some other computer Y needs to expand a file, the File System running on that computer may allocate the same block 13477 to that other file, since it has no idea that this block has been already allocated by computer X.
File Data Inconsistency:
These problems make it impossible to use Shared Disks with regular File Systems as Shared File Systems. They can be used for fail-over systems or in any other configuration where only one computer is actually using the disk at any given time. The File System on computer Y starts to process the Shared Disk only when computer X has been shutdown, or stopped using the Shared Disk.