Pretty good explanation here:
http://en.wikipedia.org/wiki/Drifting_%28motorsport%29
Cliffs notes: Instead of going around a racetrack and trying to MAINTAIN traction to get the best possible laptimes, you intentionally induce oversteer and slide sideways through a turn. For this, you get style points. (If it was based solely on laptime, then the more you drifted the worse you'd do).
An analogy would be freestyle skiing compared to downhill skiing. I'll keep my personal opinion of it to myself.