NFS is the network file system, which is used to mount or link the file system or simply folders from one linux machine to another linux machine.
This mounting makes folders and files transparently available  to the other linux machines across the network.

NFS service

NFS service is the linux service application which runs nfs server on the background, so that nfs clients can access.

How to start the NFS file system

service nfs start

or

service nfs restart

or

/etc/init.d/nfs start

or

/etc/init.d/nfs restart

But before starting nfs service you should make sure that portmap service is running, this is because nfs service depends on portmap service

How to start portmap service

service portmap start

or

service portmap restart

or

/etc/init.d/portmap start

or

/etc/init.d/portmap restart