The Elo rating system is a method for calculating the relative skill levels of players in two-player games.
Each player has a rating, which is a number. A higher number indicates a better player, based on results against other rated players.
The winner of a contest between two players gains a certain number of points in his or her rating and the losing player loses the same amount.
The number of points won or lost in a contest depends on the difference in the ratings of the players. A player will gain more points by beating a higher-rated player than by beating a lower-rated player.