Injuries suck! But it does help to know the name of your enemy and the difference between a strain and a sprain [1].

• Strain: A strain involves injury to a muscle or a tendon [2], which attach muscles to bones. Strains, aka "pulled" muscles, are caused by over stretching or tearing a muscle or a tendon.
  Common strains: hamstring and back injuries.
• Sprains: A sprain occurs at a joint and is the tearing of ligaments [3], which connect bones to bones. Ligaments are tough and fibrous bands, and can gradually stretch over time decreasing the structural integrity of a joint.
  Common sprains: Ankle and knee.

Fit's tip: To avoid both of the above injuries you should stretch [4] after working out to ensure flexible, responsive muscles so your ligaments and tendons have less chance of being pushed beyond their natural limits and tear.

Source [5]