NFS stands for Network File System. NFS is a client or a server program that runs on a computer that allows users to access files across a network and treat them as if they were located on a local file directory.
This is accomplished by a client requesting to a server, and the server allowing access to the local file system. The client machine will then 'mount' (a process by which file systems are made available to the local operating system and the user) a shared directory from the server.
The NFS protocol is designed to be independent of the computer, operating system, network architecture, and transport protocol. So a unix NFS client can connect to a windows NFS Server, like FreeNFS for example.