SPRAINS & STRAINS
A SPRAIN is an injury to the joint, effecting the ligaments and other tissues, caused by overexerting when stretching or twisting.
A STRAIN is commonly known as a "muscle pull", is caused by stretching the muscle beyond it's limits and results in tearing of the muscle.
THINGS TO LOOK FOR:

SPRAIN: Moving the victim increases pain; the skin may turn "black & blue" because of bleeding from torn tissue; can be confused with a fracture as symptoms are similar.

STRAIN: Sharp pain; the effected area is tender to the touch; a depression or bump can be seen at the effected area; extreme weakness and possible loss of use of the injured area; stiffness when moving the muscle.
TO DO:

Stay off the injured part and rest; increased movement means increased circulation which means increased swelling.
Apply ice immediately; then for 30 minutes every 2 to 3 hours for the first day or two.
When not applying ice, wrap the effected area with an elastic bandage.
Keep the effected area elevated.