Achievement

An Achievement is a reward bestowed and given to players when they accomplish notable gaming feats and obstacles in Windows Games for Vista and the Xbox 360 sets of games.

An Achievement also endows upon the player Gamerpoints that count towards a gamer's total Gamerscore. These points, while not redeemable, are intended to serve as a tangible form of special rewards for unusual or skilled actions within game play. (http://www.xbox.com/en-US/games/tips/achievements.htm)

Various achievements unlock armor types in Halo 3. In Halo 3, there are 79 achievements to unlock and a total of 1,750 Gamerscore to earn.

On September 2, 2008, A user by name Cocopjojo  post pictures from PAX showing new achievements

Comparisons and contrasts between Halo 2 PC's and Halo 3's Achievement lists
(Note that the Xbox version and specific variant of Halo 2, since it was released long before the Xbox 360, contains no achievements.)

Both games' respective lists are similar in that in both cases, achievements are split up fairly evenly between single-player and multiplayer modes.

Halo 3, however, offers quantitative rewards for finding the various skulls located throughout the game, while Halo 2 does not.

"Kill three (or four) enemies within four seconds" comprises a pair of achievements common to both titles. While Halo 2 PC refers to these achievements as "Triple Kill" and "Killtacular" respectively, however, Halo 3 refers to these as "Triple Kill and Overkill ", because Overkill is referred to as "Killtacular" in Halo 2

Halo 3 offers several achievements ("Used Car Salesman" and "Headshot Honcho", for example) that may be earned in either the single-player Campaign or multiplayer, while Halo 2 doesn't at all.

Related Links
An Xbox.com guide to understanding the concepts of 'Achievements' and 'Gamerscore'.
 * Halo 3 Achievements
 * Halo 2 for Vista Achievements