From RWP Wiki
Achievements are a feature of Xbox Live on the Xbox 360 that add points to your Gamerscore for completing tasks within a game. Achievements and your Gamerscore do not have any effect on gameplay, nor do they currently have any value on the Xbox Live Marketplace - they are merely there for bragging rights and a sense of accomplishment. When a player satisfies the conditions for an Achievement, a system notification appears, saying "Achievement unlocked." The Xbox 360's Spring 2007 Dashboard update changed the notification to also display the name of the Achievement and the amount of Gamerscore it's worth.
Most retail games have a maximum of 1000 Gamerscore to offer, and most Xbox Live Arcade titles have a maximum of 200 Gamerscore to offer. Retail games can offer less than 1000 Gamerscore: the remaining Gamerscore may be made available through downloadable expansions (as seen in Bizarre Creations' Project Gotham Racing 3). Retail games may also initially offer 1000 Gamerscore and up to 250 more Gamerscore through premium downloadable expansions. Xbox Live Arcade games are allowed to offer up to 50 additional Gamerscore through downloadable content (as seen in Gastronaut Studios' Small Arms). The amount of Gamerscore awarded by each Achievement and the number (and unlock conditions) of Achievements is up to the game's developer. Each Achievement can be unlocked only once on each Gamertag/profile and cannot be re-locked.
Electronic Arts' The Lord of the Rings: The Battle for Middle-earth II is one of the few retail games to give less than 1000 Gamerscore, offering the player only 705 Gamerscore.
THQ's Avatar: The Last Airbender - The Burning Earth has gained notoriety among Gamerscore enthusiasts for putting the player in a battle where they can unlock all 1000 Gamerscore in a matter of minutes.
Some Achievements are labeled "Secret Achievement," with a description that reads "Continue playing to unlock this secret achievement." Once the Achievement is unlocked, its description changes to a normal Achievement description.
A symbol generally accompanies a player's Gamerscore and the points given from each Achievement once an Achievement is unlocked.