Oversteer Oversteer is when the rear wheels are carving a larger arc than the front wheels or the intended line of the turn. Rear "slip angles" exceed those of the front tires. This is often described as a "loose" condition, as the car feels like it may swap ends, or be "twitchy." It causes the front of the car to turn towards the inside and the turn. This condition can be caused by "power oversteer", where you need to reduce power in order to bring the back end back into line. Understeer Understeer is when the front wheels are carving a larger arc than the rear wheels. This is often described as "push" or "pushing" - as the front end feels like it is plowing off of a corner. It causes the car to want to go straight while cornering. Further acceleration only compounds the push, as weight shifts back to the rear drive wheels off of the front turning wheels, leading to a further lessening of the car's ability to turn in. Understeer can be remedied by slight modulation in throttle to transfer weight forward to the front wheels, aiding their traction and ability to carve the turn. Many cars are designed to have a tendency to understeer. If the driver gets uncomfortable and "lifts" off the gas, that will cause the front end to tighten the curve - a relatively safer, and more predictable condition.

This is an explanation anyone can understand since I used to race, Oversteer is when you go too fast into a turn and the back of the car slides. Sometimes you spin out. Understeer is when the front of the car wants to go straight when you turn the wheel going into a corner.