An Acronym for NetworkFileSystem. Sometimes also scornfully expanded as Nightmare FileSystem.

Some useful NFS diagnostic commands:

rpcinfo -e hostname
Show what programs (and versions) are registered with the portmapper over what network protocol.
exportfs -a
Update the NFS daemon with whats listed in /etc/exports
showmount hostname
List all hosts that are considered to be mounted a filesystem on hostname
showmount -a hostname
Lists all exported filesystems on host hostname

See also some extremely good documentation on NFS issues.

Or stories about funny (or not so) resource problems.