Tennis is a sport that is played by hitting a bright green or yellow ball back and forth over a net with large rackets. It is played on a variety of surfaces, most commonly concrete (known as hardcourt), clay, and grass, as well as synthetic indoor surfaces. Tennis is one of the most popular spectator sports in the world, and large tournaments such as the US Open and Wimbledon are able to offer hundreds of thousands of dollars in prize money.
Strokes and Play
There are six basic strokes of tennis: forehand, backhand, volley, serve, lob, and overhead.
Forehands and backhands are hit along the baseline or in the no-man's-land area of the court. These strokes involve a back swing and follow-through swing over the shoulder opposite the hand being used. They are usually offensive.
A volley is a short stroke that can be hit from anywhere in the court, but is most commonly played towards the net. A volley is an offensive shot with no back swing or follow-through.
A serve is an offensive shot from behind the baseline that is played at the beginning of each point. Players toss the tennis ball above their head and whip their racquet head and hit it diagonally into the opposite service box (either the deuce or add service box). If the player fails to get the ball into the box twice they have double faulted and lose the point.
A lob can either be a defensive shot or an offensive shot. A defensive lob is hit like a normal groundstroke, but is hit very high into the air, on accident. An offensive lob goes just as high but instead the racquet whips over the shoulder of the arm being used to hit.
An overhead is a powerful offensive shot, similar to a volley, but the player hits the ball while it is high in the air.
There are four spins that players can hit the tennis ball: flat, topspin, slice, and sidespin. A flat ball is hit with no spin. Topspin is when the racquet head pushes over the top of the tennis ball, so when the ball bounces, it rotates and becomes faster. When a player slices the ball, they hit the underside of the ball. When a sliced ball bouces, it slows. When a ball with sidespin bounces, it kicks to the side of where it bounced.
The Tennis Court
There are three types of tennis courts. Concrete, often referred to as hardcourt, clay, and grass. A tennis court is 78 feet long. A singles court is 27 feet wide while a doubles court is 36 feet wide because of two 4 and 1/2 feet alleys. The net is three feet tall at the center and gradually increases in height until it becomes three and a half feet high at the two net posts. 
The Association of Tennis Professionals (ATP) determines tennis player rankings by their cumulative points total over the last 52 weeks. Players earn points by winning matches in tournaments, with more points being awarded the further a player progresses in each tournament and with more prestigious tournaments being worth more points. The rankings are used to determine player qualification and seeding in each tournament.