ELO rankings, as in chess.
You gain points by beating others. The more of an underdog you are, the more you gain but if you were overwhelming favorites you only gain 1.
You also can gain points by drawing against people higher rated than you. If they're rated around the same you get 0 points. And if you draw with someone far below you you can lose points.
And finally you lose points when you lose. If you were an overwhelming favorite you lose way more than if you were an underdog.
At its core the amount won/lost is driven by a formula that someone can arbitrarily adjust - they list their K value here http://clubelo.com/System