Strangely for a martial art, judo in English means the 'gentle way'.
Players are split into weight categories, so small judokas battle small judokas and larger
ones fight each other too.
It's supposed to be a defensive sport, where judo players use their opponents strength and movement against them.
The idea is to throw your opponent to the floor and hold them for 25 seconds so they can't move, or force them to submit because they can't move.
The first person to win 10 points wins the match.
To win all 10 points in one go, called Ippon, an opponent has to be thrown, but it must be done just right. If even a little bit is wrong it's called a waza-ari and earns only seven points.
Judoka can also earn points for other throws, with a koka worth five points and a yuko worth three.
Matches last five minutes for men and four minutes for women and if neither competitor has earned 10 points in that time the fighter with the most points wins.
Bizarrely, winning a match by choking your opponent or breaking their arm is within the rules.
Who are the British hopes for Athens?
Craig Fallon in the men's under 60kg category is a top medal hope, as he won silver at the world championships.
Kate Howey won silver at the Sydney Games and will be hoping to go one better at her fourth Games in Athens in the women's under 70kg category.
Who are the big stars?
Kosei Inoue is often thought of as the finest judoka in the world. A three-time world champion and Olympic gold medallist in Sydney, he is expected to win again.
Ryoko Tani in the women's event is perhaps an even bigger favourite. She has won the world title an astonishing six times, won an Olympic silver aged 16 in 1992 and won gold in 2000.