NFS (Network File System) is a protocol that is used to serve and share files on a network. Similar protocols include SMB (Server Message Block) and AFP (Apple Filing Protocol).
iSCSI is a transport layer protocol that describes how Small Computer System Interface (SCSI) packets are transferred over a network. It works by transporting data between a server and a storage device.
NFS and iSCSI are fundamentally different ways of data sharing. NFS is built for data sharing among multiple client machines. On the opposite end, iSCSI is a block protocol which supports a single client for each volume on the server. While it does permit applications running on a single client machine to share remote data, it is not the best for sharing data across machines. It is possible to run iSCSI in a shared multi-client environment but requires designing a distributed file system that accesses data from a block server.