Arpad Elo developed the self-named elo rating system, a system designed originally to fairly calculate the overall rankings of chess players. Elo has also seen exposure in different sports and games, branching off to areas like Football, Scrabble, or Backgammon, and video games such as Guild Wars, Starcraft 2, and League of Legends.
In a roughly abbreviated explanation, players under the elo system are placed at a specifically middle rating, say 1200 in League of Legends. With subsequent games, players can increase their score from victories or decrease their score from losses. The fine tuning of points earned when winning or losing a game are usually made separately for each type of game, as different mechanics are involved.
A: Man, I've been losing so many games, my elo's almost dropped to three digits!
B: That sucks, there's not much you can do besides get better.