If I had my videocam now I'd make a sample movie, It's hard to explain because my English is so bad.

But I'll try...
Neutral Steer(Four-Wheel-Drift): Both front and rear tires are sliping in the same angle.
Oversteer: The rear tires have a larger slip angle than the front tires, so the car fishtails through the corner.
Understeer: The front tires have a larger slip angle than the rear tires, so the car tends push straight ahead to the outside of the turn.
Powerslide: It's an oversteer caused by excess of throttle in a rear or all wheel drive car. Normally happens in the acceleration part of the corner.
Drifting: This term has notting to do with the four-wheel-drift, and it's was not created by japanimation crowd either. It's rather a driving style that evolved from what is called
Touge Running in Japan. In the beggining, it was considered dangerous to lose the rear wheels traction during the corner, so the drivers tried their best to go as fast as possible without losing control, eventually it became cool to lose the rear wheel traction and regain it quickly, it showed the driver had great skill. Then it kept evolving and drivers would purposefully lose traction way before the corner and take more than one corner without regaining traction on the rear wheels. That's what's some people call "Exhibition Drifting", the more sideways the better, speed is not really that important.