provides services related to the session layer of the OSI model allowing applications on separate computers to communicate over a local area network
As strictly an API, NetBIOS is NOT a networking protocol (neither network layer nor transport layer)

Services

NetBIOS provides 3 distinct services:

Name Service (NetBIOS-NS) - for name registration and resolution
Datagram Distribution Service (NetBIOS-DGM) - for connectionless communication
Session Service (NetBIOS-SSN) - for connection-oriented communication