The purpose of the warmup is to gradually prepare your body for the activity.
If performed properly you are less like to get injuries and will perform
more effectively.

The three parts to a warmup are:
Slow Rotations - gently move body parts thus increasing blood flow to muscle
areas and joints
Body Temperature - perform low intensity exercises so that more blood enters
the muscles
Flexibility - once the muscles and joints are hot they are ready to be stretched
The stretches should be slow and gentle - they shouldn't be painful.