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.
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.
To serve in tennis, is to hit a tennis ball with your racquet, aiming to score against an opponent.
A big serve is when you the serve with a lot of power.
A tennis serve is out if it hits the green without hitting the main court, or if it hits the net
It would called as 'let' and server gets another chance to serve.