Oversteer & Understeer
Law Of Physics: Forces
You have to remember that in racing you are dealing with the law of physics. It is exactly this law that racers seek out to challenge. It takes a long time to really understand and know how to anticipate the forces working on a moving vehicle.
How all of these forces are working on your car will ultimately determine the behavior of the most important element of your vehicle... the tires! In the end its the tires that decide how your car will react to your input.
You can have all the horsepower you want, all this power becomes useless if you can't put it down on the ground.
THE FAMOUS FRICTION CIRCLE
Latitudinal and longitudinal forces are working on the tires in every corner. If you go around a corner it is virtually impossible to get 100% grip.
Even if youre going around a corner at moderate speeds it might seem like you are having 100% grip, but in fact you are sacrificing some of the available grip for your steering movements.
So when you are cornering there always exists some slip angle, no matter how small this might be.