Achievements are available in all Xbox 360 Games and XBLA Games. Each Achievement is worth a certain amount of Gamerscore, generally, the harder the achievement, the more Gamerscore you earn. To unlock an Achievement, you must complete the requirements displayed in the Achievement description, thus earning the Achievement and it's respective Gamerscore. Each game has a certain amount of Achievements, which when completed will add the game to your completed games list. You can also increase the amount of Achievements available in a game by downloading DLC.