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 opponent's 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.