Achievements are the greatest innovation of the Xbox360. Not the graphics, the wireless controllers, the updated Live service. No. It's achievements. People are mad for them. I'm mad for them. There are games that get bought just because they have easy achievments and can inflate scores.
For those who don't know, an "Achievement" is a little badge you are awarded in Xbox 360 games for completing a certain task. On the boring side, you can get achievements for completing a game on different difficulty levels or for collecting 100 Whatsits or killing a million Stormtroopers or whatever.
On the more interetsing side, you can get achievements in some games for staring at someone's crotch, for ironically taking a picture of a serial killer who photographed his victims, or in the case of Halflife2: Episode 2 for carrying a garden gnome throughout the entire game and then launching it into space.
More details at the link.