Per the Boston Globe:

The Globe's Shira Springer, citing NFL sources, reports today that Patriots quarterback Tom Brady suffered a torn anterior cruciate ligament (ACL) and torn medial collateral ligament (MCL) in his left knee.

Medical tests on the knee, including an MRI performed Monday, indicated no damage to other knee ligaments and no torn cartilage. The typical course of action for such an injury is to wait 4-6 weeks for the MCL tear to heal, then reconstruct the ACL. Brady will likely undergo surgery in approximately one month and face 6-9 months of recovery and rehabilitation, barring any complications.

For more, check out Springer's complete report.

Now the important question: What are the ACL and MCL? Wikipedia to the rescue.

The anterior cruciate ligament (or ACL) is one of the four major ligaments of the knee. It connects from a posterio-lateral part of the femur to an anterio-medial part of the tibia. These attachments allow it to resist anterior translation of the tibia, in relation to the femur. More specifically, it is attached to the depression in front of the intercondyloid eminence of the tibia, being blended with the anterior extremity of the lateral meniscus. It passes up, backward, and laterally, and is fixed into the medial and back part of the lateral condyle of the femur.

 

The posterior cruciate ligament (or PCL) is one of the four major ligaments of the knee. It connects the posterior intercondylar area of the tibia to the medial condyle of the femur. This configuration allows the PCL to resist forces pushing the tibia posteriorly relative to the femur.

Brady can expect around 9 months of rehabilitation time.