There are hundreds of rules to the game of tennis. There are additional variations depending on the governing body (USTA, ITF, ATP, WTA, ITA, NCAA) of the particular event. In the United States, the USTA (United States Tennis Association) has set some very specific rules that apply to tournaments and leagues. Many of these rules are followed in nearly all instances of play.
A singles court is 78 feet long by 27 feet wide. A doubles court is the same length, but 36 feet wide. The court is divided down the middle (horizontally) by a net, which is suspended by a metal cable or cord through two net posts. There must be lines at the sides of the courts (sidelines) and at the ends of the court (baselines).
The rules for scoring tennis are a bit strange to a novice. Instead of "zero," the term "love" is used for no points. If a player achieves one point, it is considered "15"; two points are "30"; three points are "40" and a fourth point gives the player a game.
There are exceptions to the "win at 40" rule. A player must win a game with two more points than her opponent. If both players have 40 points, it is "deuce" and one of the players will need two consecutive points for the game.
Winning a Set
In order to win a set in tennis you need to win six games, but like with the points, you must win by two. If one player wins six games and the other has four or less, the player with six has won the set.
If both players reach six games, a tiebreaker is played to determine the winner of the set. A tiebreaker is won by the first one to reach seven points, again with a lead of two. During a tiebreaker, the players will switch sides of the court every time the score is divisible by six (i.e. 3-3, 4-2, 6-6).
Before serving a point, both feet must be behind the baseline (it is called a "foot fault" if a player's foot touches the baseline during the serve). The server must release the ball with his hand and swing at it with the racket. That must be done before the ball reaches the ground.
A serve is good (in play) if it lands within the box diagonally across from the server. The receiver can then return the ball anywhere within the court lines. If the server does not get the ball within the box on two tries, the receiver gets the point.
There are two instances when a let, or permission to replay the point, is given. The first is when the server hits a serve that touches the net, but lands within the receiver's box. The second is if there is interference in play (like a ball from another court rolling on).
There are some other instances in addition to missing serves and not returning the ball within the court when a player can lose a point: if the ball bounces twice before it is hit; the ball hits a permanent fixture (i.e. lighting); the ball is hit with a racket twice; the ball touches the player or the player's clothing.
With a few exceptions--college tennis, some team events--coaching is not allowed when a player is on the court. This includes the ban of verbal and physical signals between the coach and the player.