Well Linux clustering and SANs are two different monsters(although they interact commonly in some setups). I am assuming you are referring to clustered file systems, if not be a bit more specific.
What are you having troubles with on SANs, its kind of a generalization, there are the basic fundamentals of what a SAN is and then there are the other features that providers have built into their SANs that can be misconstrued as to being a fundamental feature of SAN topology.
is a good place to get some basic info on what a SAN is and what the difference is between a NAS.
At the end of the day its basically a device or block level remote storage, usually presented over a fibre channel. The server sees these as physical disks the same way it would recognize a hard drive in the system. How the LUN is presented to the system is dependent upon the SAN you are using, Hitachi is slightly different from EMC and so on.