Achievements are synonymous with Gamerscore on the Xbox 360.
When you unlock an Achievement, you get Gamerscore. To unlock an Achievement, you need to fulfil a specific task.
Achievements vary in difficulty to unlock; some take little to no effort whatsoever, whereas some are nigh impossible to achieve.
Example of an easy Achievement to unlock:
Gears of War: Prison Breakout -
Completed tutorial level on any skill level
Example of a hard Achievement to unlock:
Gears of War: Seriously ... -
Kill 10,000 people in versus ranked match total
Getting more popular in video games today, an achievement is a feat that the game records and displays for others to see that you have accomplished such a task. They are usually rewarded with points that have no real value other than to enlarge one's e-peen. Achievements were originally started by Xbox 360 Live. Other games like World of Warcraft have taken on this addition as well.
"Dude check out this achievement I just got for falling 30 feet off this cliff and dying!"
Successfully reaching OG during a sexual act.
"Britney Spears! Britney Spears! Britney Spears! Brit-........... uhh..... achievement!"
"Oh that's just sad."
"For ****'s sake! I'm in here! Get out!"