an acronym for Network File Services - a file sharing protocol
NFS is a proven standard protocol, still being improved (it had it's 4th version released in 2003, with a 4.1 edition from 2010). It is an opened standard (RFC) and anyone can help improve it or implement it.
NFS is considered fast in comparison to other network protocols like SMB/CIFS (Windows world) or AFP (Mac world), this is not alway true but one can choose to have NFS on top of UDP instead of TCP. UDP can have less overhead in some usage than TCP, thus being faster, however this come at the cost of less reliability and possible data corruption.
NFS lacks proper security, it relies mainly on IP address and UID (user ID) for security. NFS v4 has added Kerberos authentication to minimise this major drawback, although setting up a Kerberos environment is not that easy. Previous NFS release should not be used to access sensible data over unsecure and untrusted network.