The knee is a hinged joint allowing for flexion and extension only.  However, rotation of the Tibia occurs which allows for quick cutting and other agility movements to be made.  The ligaments of the knee help to prevent any movement that the knee is not designed for.  The muscles above and below the knee joint are the prime movers.  These muscles are what allow us to walk, run, jump, climb, etc.  If you injure your knee, you lose the ability to perform these activities.
An ACL injury normally occurs from some type of excessive rotational stress applied to the knee or a hyperextension injury with a quick movement or sudden stopping.  Remember your knee is only designed to bend and straighten.  When an ACL injury occurs, the athlete may feel a pop within the knee.  They will also experience a feeling of the knee “giving out”.  The phrase “Trick knee” has long been associated with an ACL injury.  The knee joint will feel unstable and not allowing the athlete to cut or performed agility motions.
Other symptoms may include immediate swelling, an inability to fully straighten or bend the knee.  The most common symptom is the feeling of the knee “giving out” or feeling unstable.  It is not uncommon for other structures in the knee to be damaged at the same time that one injures their ACL.  It is common that when an injury occurs to the ACL, either the medial or lateral meniscus may be torn along with articular cartilage.

ACL Injury Anatomy