Clusters are file system blocks, in Microsoft-speak.
Volumes are high-level file system containers, usually mounted partitions. IE, the volume "C:\" is usually the 2nd primary partition on a drive, while the first partition is hidden from you. They are usually mapped to each other on a 1:1 basis, but not always. They always appear to be, to a user, though.
Fragmentation...dude, Google it. It's a simple, yet emergently rather complex, issue.