ACL stands for anterior cruciate ligament; this ligament runs diagonally across in the middle of your knee. When the ACL is injured, the surrounding ligaments and cartilage are often sprained as well. While the ACL could tear only partially, this is rare; more commonly there is a near complete or complete tear. This injury is most common in athletes who play football, basketball and soccer for a reason-ACL tears happen as a result of rapid direction changes, improperly landing from a jump, stopping suddenly, and direct collision with another player, all of which are common aspects of these three sports.
When the tear happens you may hear a popping noise and/or feel your knee give out from under you. Within 24 hours you'll begin to feel severe pain and swelling in your knee and aching or tenderness in the joint.