NetBIOS stands for "Network Basic Input/Output System." NetBIOS was introduced in 1983 by IBM as an improvement to the standard BIOS used by Windows-based computers. The BIOS provides an interface between computer's operation and hardware. As the name implies, NetBIOS adds support for networking, including the ability to recognize other devices connected to the network.
NetBIOS provides an API (Application Programming Interface) for software developers to use. This API includes network-related functions and commands, which can be incorporated into software programs. For example, a programmer can use a prewritten NetBIOS function to enable a software program to access other devices on a network. This is much easier than writing the networking code from scratch. In other words, NetBIOS prevents programmers from having to "reinvent the wheel" just to get their program to connect to a network.
NetBIOS session service (NBSS) is a method to connect two computers for transmitting large messages or heavy data traffic. Because the NetBIOS session service is involved in traffic generation and forwarding, TCP port 139 is used. The NetBIOS session service is mostly used for printer and file services over a network.