Also called "loose". Oversteer basically happens when the rear wheels don't have enough traction for one reason or another.
Oversteer is not always bad. In racing circles, you often hear "oversteer is faster." Controlled oversteer can help move the rear of the car around obstacles (like cones in autocross), or help navigate very sharp corners.
In car setup, you can adjust towards oversteer by increasing the rigidity of the rear sway bar; this forces the rear wheels to do more work in cornering, making them more prone to losing traction.
While driving, oversteer often happens when braking in a corner. Braking causes weight transfer to the front wheels (think about it), meaning there is less force pushing the rear wheels against the pavement. They may have been happy before, but the sudden loss of weight makes the rear end step out and try to pass the front. This is why you should always, always brake in a straight line.