NFS stands for the Network Filesystem. It is not really part of the real filesystem, but can be used to add parts to the mounted filesystem.
Large Unix environments often times share the same programs, sets of home directories, and mail spool. The problem of getting the same copy to each machine is solved with NFS. We can use NFS to share one set of home directories between all of the workstations. The workstations then mount that NFS share as if it were on their own machines.
See Section 5.6.2 and the man pages for exports(5), nfsd(8), and mountd(8) for more information.