Shared disk is simply disk which is accessible from more than one computer at the same time.
Commonly these arrangements are fiber channel, and can include redundant FC interconnect cards, and fiber channel switches, so that they can be made with no single point of failure.
Examples include IBM's FAStT(DS4300) and ESS (Shark) lines of fiber channel disks.
Many other manufacturers also make similar products.
The only constraint for this kind of "simple" configuration is that they be accessible from either side without any kind of special command having to be issued other than a mount command.