Short for Network File System, but there are others. Invented at Sun. Supports a large fraction of UNIX filesystem semantics (larger than the other networked FSes) but definitely not complete.
Tcl and NFS
One issue, at least up until 8.5a1, is that a fair number of tests in the core test suite fail when run in an NFS-mounted directory. This is because of the fact that NFS only supports a subset of the classic semantics.
[Other issues?]