NetBT is an acronym that means "NetBios over TCP/IP". NetBios stands for "Network Basic Input Output System". TCP/IP stands for "Transport Control Protocol/Internet Protocol".
NetBios is a session layer API. As such, it can run on a variety of transport protocols, such as TCP/IP, NetWare, VINES, etc. An API is an application-programming interface. The NetBios API defines an interface that programmers can use to write code that performs various networking functions. NetBios functions enable a programmer to write programs that can log in to a remote machine, connect to a shared drive on a remote machine, get a lists of sessions and users from a remote machine, and many other networking tasks.
TCP/IP is commonly referred to as the "Internet Protocol". It is responsible for sending and receiving data to and from your machine and the Web Servers, Mail Servers, and DNS servers you use on the Internet. It is the protocol that makes it possible for you to use the Internet.