Gamerscore

A Gamerpoint is an award given in a numerical value shown on your gamertag. You attain gamerpoints after completing special tasks or milestones in a game, otherwise known as achievements. In some games, gaining a certain amount of gamerpoints unlocks something in the game, such as armor or new weapons, or gaining a certain number of gamerpoints can also gain achievements in some games. While most games have 1000 gamerpoints in total, some games (eg. Xbox Live Arcade games) have far less than 1000 gamerpoints. Also, some games are not shipped with 1000 gamerpoints, but offer achievements as downloadable content. The total amount of Gamerpoints is called the "Gamerscore" by most people. Normally a game has 1000 gamerpoints but if an expansion pack is released more can be added on, as per the case of the DLC achievements that shipped with the Legendary Map Pack and the Mythic Map Pack. The Armor Permutation Security can only be unlocked by collecting 1000 Halo 3 Gamerpoints. Other Permutations can only be achieved by completing achievements.

Note: These are not to be confused with "Microsoft Points", which are a stored-value system used for purchasing content on Xbox Live.