Что такое NET?

Смотря в каком контексте. Вообще Net - это сеть. Если вы имеете ввиду .NET, то это библиотека классов.

Net-
1. a bag or other contrivance of strong thread or cord worked into an open, meshed fabric, for catching fish, birds, or other animals: a butterfly net.
2. a piece of meshed fabric designed to serve a specific purpose, as to divide a court in racket games or protect against insects: a tennis net; a mosquito net.
3. anything serving to catch or ensnare: a police net to trap the bank robber.
4. a lacelike fabric with a uniform mesh of cotton, silk, rayon, nylon, etc., often forming the foundation of any of various laces.
5. (in tennis, badminton, etc.) a ball that hits the net.
6. Often, nets. the goal in hockey or lacrosse.
7. any network or reticulated system of filaments, lines, veins, or the like.
8. any network containing computers and telecommunications equipment.
9. the Net, the Internet.
10. Mathematics. the abstraction, in topology, of a sequence; a map from a directed set to a given space.
11. Informal. a radio or television network.
–verb (used with object) 13. to cover, screen, or enclose with a net or netting: netting the bed to keep out mosquitoes.
12. to take with a net: to net fish.
13. to set or use nets in (a river, stream, etc.), as for catching fish.
14. to catch or ensnare: to net a dangerous criminal.
15. (in tennis, badminton, etc.) to hit (the ball) into the net.

Сеть

Сокращение от интернет. По-англ. нет - сеть.