the umpire or the returner
In professional tennis, there are line judges for that. In non-professional tennis, however, a server can call his or her own serve out. Most do not because it is a point against them, even though the opponent thought the serve was in.
Because you hit the ball with some kind of racket and that is why you call it a serve.
a service fault in tennis is when you either when you serve and you don't get it over the first line OR you get it out
It is called a 'Let'
a serve that you cant get back over the net e.g. its to powerful or you cant get to it in time
Do you mean "let"? If so, a let is when a serve touches/nicks the net AND lands in on a player's serve. People call "let" so the server knows their serve hit the net. If a server hits a let, they get to redo their serve.
There ISN'T anything called a "backhand serve" in tennis.