understeer - you're turning the steering wheel, but you continue to plow straight

oversteer - you're turning the steering wheel, and the back end comes around

I've seen this from another board

 

understeer - also known as pushing, ploughing, etc. its common on front and all wheel drive cars, and occours when the front tires don't have sufficient traction to maintain the line though a corner, and as a result the front end continues going straight rather than turning as directed by the driver.

oversteer - this is caused by the rear tires loosing traction in a corner, and the result is the back end of the car attempting to beat the front through the corner unless the slide is controlled. it it looked cool, it was probably oversteer. its common in rear drive cars with lots of power, and its what drifting is all about.

 

get a rear sway bar to get rid of some of the understeer in you car if your stock.