Before The Serve, 2010 US Open

A legal serve occurs when the ball lands in the cross-court service box. If the ball lands outside the box, it is a fault. If two faults of any kind are given in a point, it is a double fault and the point is awarded to the receiver.
A foot fault takes place when the server's feet get into an illegal position while serving. A server's feet must not touch the baseline or center line until the ball is hit. The server is also required to stay roughly on the same position to prevent the opponent from being misled as to where the serve will originate. Running or walking while serving is not allowed.
If the server tosses and attempts to swing but misses the ball, it is a fault. However, if the server is unsatisfied with his toss, he can let the ball fall or catch it (with his hand or racket) as many times as he wants without getting a fault.
A service is called a let when the ball hits the net cord but still lands in the service court. It is a void and the serve is replayed (no fault is given). A ball that hits the net cord but lands outside the service box is still a fault.