Oversteer & Understeer
A tire can only sustain a certain amount of force(s) until it loses traction; it starts to slip (Hence the term slip angle). And when it starts to slip speed is lost.
Slip angle is a term that tells you how much a tire is skidding (sliding/slipping). A bigger slip angle (measured in degrees of course) means that the tire starts to slide more...
Lets define what slip angle is:
When a tire is cornering the contact patch has to resist the forces of friction between the rubber and the road surface. Due to the elastic nature of the tire the tread is distorted in the way that forces are working at that particular time. Its resisting the turning action, thus pointing a different way than the actual desired path.
This angular difference is called slip angle.
Here is a good illustration to help you better understand slip angle.
A BIG DRIFT ANGLE IS A GOOD THING! :)
Tires seem to operate at their peak performance when they are under a few degrees of slip angle, they generate the most grip at that particular slip angle. For race and high performance tires this optimum slip angle is around 6 to 10 degrees while this number is a little lower for street tires.
Due to low traction surfaces rally drivers reach even bigger angles. In drifting you probably see the biggest slip angle of all motorsports, sometimes as high as 40 degrees!
If you are cornering and the slip angle is below its optimum range the tire is considered to be under-used. If its above this range the tire is being over-used. The trick is to stay within this optimum range so you use the tires to their fullest potential!