Is this a FPS engine or a game?

Nexuiz is a FPS game, that uses DarkPlaces engine

You mean Nexuiz? If you took a look at www.nexuiz.com or do just a little research you would see that it's a game.
DarkPlaces is the engine.

Ok but can some one tell me how to work with that angine? il give him a credit...

I don't understand what you want, you want to code for the engine? If so, the language is QuakeC.

What i want is to know how to build a game like this.
I know the basics of C# if i need it,
How can i learn the language and where?

Nexuiz is open source both for code and content. Extract the source and look through it. I'm afraid no one is going to give you a walkthrough of how to build a game. Look at the individual parts of the content and see what you can make yourself.

http://ouns.nexuizninjaz.com/dev:progra ... troduction

Use search next time.