In tennis, an official is a person who insures that a match or tournament is conducted according to the International Tennis Federation (ITF) Rules of Tennis and other competition regulations.
At the highest levels of the sport a team of up to ten officials may be on court at any given time. These officials are broken up in to categories based on their responsibility during the match. Contrastingly, many tennis matches are conducted with no officials present.
The Chair Umpire "is the final authority on all questions of fact during the match." Questions of fact include whether or not a ball was in, the calling of a service let or the calling of a foot-fault.
The Line Umpire "calls all shots relating to the assigned lines." Line umpires work on court as part of a team of between three and nine line umpires. Each line umpire is assigned by the chair umpire to one line or, in the case of a short handed crew, a position in a system. For example, a line umpire on the receiver's side may have to cover the center service line then, following the serve, move to one of the sidelines.