There are two faults that can occur in a tennis match, a service fault and a foot fault.
Source:
ITF Rule 18: "During the service motion, the server shall not:
a. Change position by walking or running, although slight movements of the feet
are permitted; or
b. Touch the baseline or the court with either foot; or
c. Touch the area outside the imaginary extension of the sideline with either
foot; or
d. Touch the imaginary extension of the centre mark with either foot."
ITF Rule 19: "The service is a fault if:
a. The server breaks Rules 16, 17, or 18; or
b. The server misses the ball when trying to hit it; or
c. The ball served touches a permanent fixture, singles stick or net post before it hits the ground; or
d. The ball served touches the server or server’s partner, or anything the server or server’s partner is wearing or carrying. "