Source: http://www.usjudo.org/judocompetition.asp

A judo match takes place between two players, on a mat, and is watched and scored by referees.

Within a judo match, the objective is to score an ippon, which is akin to a pin in wrestling or a knockout in boxing. When an ippon is scored, a match is over. There are three ways to attain an ippon:

Knock the opponent over so that he lands hard on the back (a throw).

Hold the opponent on the back for 25 seconds.

Make the opponent submit by a strangle hold (choke) or an armlock.

There are hundreds of variations of throwing techniques. Some throws use mostly legs, some mostly arms, some a combination of arms, legs and torso. Some students throw their opponents over their own hips, shoulders or backs. They can also sweep the opponent's feet out from under him or drop down and throw the opponents over their own fallen body.

To pin an opponent, a student can press down from a facedown or side-down position (generally controlling the head and an arm or leg) so that the opponent's back or a shoulder is on the mat.

To obtain a submission, pressure can be applied directly on the elbow of a straight arm, or an arm bent at a right angle can be twisted in either direction (armlock). To strangle, or choke, pressure is applied to the sides of the opponent's neck (not the windpipe) by one or both forearms or by using the opponent's own collar. Strangle holds are only allowed on players 13-years-old or older. Arm locks can only be used if the player is at least 17-years-old.

Although strangles and armlocks may seem dangerous, the players are trained to know when they are in danger and will submit by tapping either the mat, or the opponent, twice, before any damage is done. The referees are extremely alert when one player attempts to apply a choke or an armlock. If the referee thinks the technique is about to cause serious injury, he can stop the match and declare a winner.