>What exactly is the volume serial number?
>Is different for each hard drive or CD Rom?
>Is it Unique between computers?

It's a random number created when formatting a disk. This DWORD number is written in boot sector of a disk to distinguish between disks. This number may be changed by utilities, not created when formatting in early versions of MS DOS, not returned by GetVolumeInformation() for remote drives on Windows95/98 and may be not unique between computers.