Those are pretty broad topics. You'll need to do a good bit of research on each of them before you take the test unless it's a very easy test.
RAID is a method of spanning data across multiple hard disks, and usually you also implement redundancy, so that if a drive fails, you can replace it without losing all of the data.
iSCSI is a method for using the SCSI protocol over an ethernet network. I believe SCSI is basically a parallel interface for attaching disk drives to a computer.
With iSCSI, the initiator is usually that computer that is connecting to the iSCSI host. The initiator mounts a remote hard drive (or raid array) over the ethernet network and treats it as if it were a hard drive directly plugged into the computer.
SAN means Storage Area Network. Start with the wikipedia page on SAN networks, it covers a lot of it. I'm not sure what you mean by 'environments' probably the common types of san infrastructure or SAN switches? unfortunately I don't know a whole lot about SANs.