In toribash you start with an elo of 1600. When you win against an opponent you gain elo, if you lose a match, you lose elo. The higher the difference between the elo of you and your opponent is, the more/less elo you will win/lose.
Example: If the elo of both you and your opponent is 1600, the winner will gain 1 elo and the loser will lose 1 elo. If the elo of your opponent is 1650 instead, he will lose 3 elo and you will gain 1 elo if you win and vice versa. (as already explained in the wikipedia link Kirito posted)
Those are the basics. But as far as I know, it is way more complex than that. The system always tries to get everything in order, so that the player with the highest elo is rank 1 etc. But the problem is that you can only win/lose ~3 ranks per match. So if you gain 3 elo in one match, you actually overtake thousands of other players at once, meaning you don't gain the rank you would deserve corresponding to your elo. The system tries to compensate that by sometimes swapping the ranks of two players. If your opponent has worse elo but a better rank than you, the system thinks his rank is too good for his elo, so it will give the rank of your opponent to you to bring you both closer to the rank you would deserve. But of course, nothing of that is perfect and so you will probably never gain your "true" rank (the rank you would get if all elos would be in perfect order) and so the system will go on and on swapping ranks and stuff, demanding perfection it will never be able to achieve.
At least thats what I know of how this system works, I might be wrong about something and you might not even understand what I even tried to say in my huge wall o' text, but I hope I was able to help in some way.
Last edited by Zockinator; Aug 21, 2015 at 02:33 AM.