Location: The wheels on your car.
What does it do? Creates friction between the road surface and your wheels so that you have the best control over your car as possible.
Replacement: Tires should be replaced every time you see they are...
If your tires match any of the above problems, it's a good idea to replace them. A worn tire is never a safe thing to drive with, it may unexpectedly blow out and cause an accident which could end your life.