It's basically what is sounds like. The famous card game UNO in Xbox Live Arcade form. You can find other versions for PS3, Wii, iPod Touch, iPhone, DSi and practically every other system made in the past five years.
It plays well with very little trouble, and the online play work nice. It's really cool that it's a very social game. Most of the people who play do bring a headset along.
I don't mean the guys who RAGE on Xbox Live either. You'll find your fair share of jerks on the game for sure, but this is one game where most players aren't rude.
For the lonely guys, plenty of women play UNO on 360 as well.