The ACL and PCL are short for the Anterior Cruciate and Posterior Cruciate Ligaments. Both are located in the knee joint, attaching the bottom of the femur to the top of the tibia bone. See the picture below for a better representation of these ligaments. As you can see the ACL is located in the front, or more anterior, and the PCL is located in the back, or more posterior (hence their names). The ACL is the ligament of the knee that keeps the tibia (lower half of the knee) from translating forward, while the PCL keeps it from going backwards. When a person tears their ACL or PCL they usually hear a popping noise first followed by almost immediate inflammation of the joint. If these symptoms are not present, a doctor can always test the integrity of the ligaments by performing anterior and posterior drawer test (basically moving the joint back and forward to see if there is abnormal joint play).
So why is this injury so common among athletes? The ACL and PCL are criss crossed from the lateral to medial side of the knee and medial to lateral side of the knee respectively, making any kind of torsional strain on the knee joint bad for these ligaments. Now add in players who have to pivot to catch a ball and are hit at the same time. This is why it is so common for football players to sustain these injuries.
The best way to help prevent these injuries are to avoid the motions that stress the ligaments and to also strengthen the surrounding musculature. Like with all injury prevention, remember to stay active but be aware of surroundings and try to avoid putting unnecessary
stresses on a single joint at any one time.