The ACL (Anterior Cruciate Ligament) is a ligament that helps connect the femur (thigh bone) to the tibia (shin bone). This ligament helps to prevent the two leg bones from sliding forward/backward on each other creating a shearing effect in the knee. The ACL also stabilizes the knee during rotation when the foot is planted. Non-contact sport injuries are the most common cause of ACL tears. When the knee is locked and rotates while the foot is firmly planted it can cause a partial or complete ACL tear. Another common sport injury that results in an ACL tear involves falling on the knee causing one of the leg bones to shear against the other, thus the ligament rips. 

Source: medicinenet.com