Tennis is a very social activity and can be played indoors or outdoors, in teams with families and friends. Wheelchair tennis is a cross-disability activity. Proficient wheelchair users can play and actively compete against non-wheelchair users. Players can play the game on any regular tennis court and no modifications are required for tennis rackets and balls.
A wheelchair tennis match is played between two or four athletes with loss of motor function in one or both lower extremities. There is a special class for people with quadriplegia, or athletes with loss of motor function in the lower extremities and functional disability or amputation in the upper extremities. To score a point, wheelchair tennis players have to hit the ball with their rackets so that it lands on the opponent's court without the opponent being able to hit it back. The winner of the match is the athlete or pair to win three "sets" of six "games" each (best-of-three set match).
(From http://en.paralympic.beijing2008.cn/sports/wheelchairtennis/index.shtml ).