I recently watched a Judo fight and I was confused about the scoring system. How exactly can a fight be won?
There are (as of 2018) four ways how a fight can be won:
Ippon - An Ippon ("full point") is awarded, when either the opponent is thrown on the back with force, control and speed or when the opponent is fixed in Osae-komi for 20 seconds or when the opponent surrenders after an arm lock or a strangulation. Every Ippon ends the fight immediately.
Waza-ari - If you perform a technique, that does not fully match the criterias for Ippon, you will be awarded an Waza-ari ("half point"). Two Waza-aris result in an Ippon ("Waza-ari-awasete-Ippon") and end the fight immediately. Also the fighter with more Waza-aris wins, when the fight time (usually 4 mins) is over.
Fight end - In case of a draw, the fight will continue in Golden Score-mode. The fighter that scores the first score will immediately end the fight and be declared winner.
Some smaller tournaments don't have Golden score but use a Hantei instead. In this case the referee decides who fought better and declares this Judoka the winner.
Hansoku-make - For major or repetitive rule breaks, actions against the spirit of Judo or the performance of techniques that put the fighters health in danger a Hansoku-Make can be given. This penalty is a disqualification and ends the fight immediately. The punished fighter may or may not (depending of the circumstances) continue to fight in the tournament.
More information can be seen in a video published by the IJF (international Judo federation) to inform about the 2018 rules: