Any particular method for handling files of data is called a filesystem. Athena currently uses AFS, the Andrew Filesystem, as the filesystem for all user home directories and most of the other lockers.
AFS is a distributed filesystem. This means that the files stored using AFS are located on a remote server, so you can access them from any Athena workstation. In fact, the owner can make the files available from any computer running AFS in the whole world.
You don't have to attach AFS filesystems; they are always available from the workstation under the directory /afs. Another major difference between AFS and other distributed filesystems is that groups of AFS files can be replicated onto multiple servers in such a way that makes the failure of one server unnoticeable to a user. In addition, files and directory structures are cached locally to minimize network traffic to the server. This makes access to AFS files very fast after the first time a file is read.