The two terms, oversteer and understeer, are two vehicle dynamics used to describe a vehicle's sensitivity to its steering. Oversteer is what will occur when a car steers by more than what is commanded by the driver. The understeer will happen when the vehicle drives less than the amount controlled by its driver.
Most experts will define oversteer and understeer based on the changes in the steering angles associated with various changed in the lateral acceleration that occurs after steady turning tests. Most car enthusiasts mostly use this terminology more in blogs and magazines to describe the car's responses to the steering in all types of maneuvers.
The difference between oversteering and understeering will be determined by the way the steering behaves. They will also be determined by the small differences in the steady-state conditions where the vehicle will follow constant radius paths with a constant driving and steering angle.