There is quite a bit of confusion on what exactly the rotator cuff is and does. Sparing the detailed anatomy (if you want that, see the links below) the rotator cuff is comprised of four muscles and their tendons. These muscles are anchored on your shoulder blade and attach to the top of your upper arm bone. A common analogy used to describe the shoulder joint is a golf ball on a golf tee. It is a joint that gives a lot of motion but the sacrifice is stability. An that's where the problems arise.
Patients with rotator cuff injuries can come from a variety of walks of life, but people with jobs working overhead and athletes are the most prone to injury. The most frequent injury is tendonitis, an inflammation of the tendon, which can be quite painful and limiting. Tears are usually of the uppermost muscle of the cuff and generally are of the "wear and tear" type versus traumatic injury. A very interesting study revealed that a surprising amount of the population may have a tear and not have any pain or symptoms. We have worked with many patients that had significant and even full tears that have rehabbed well without surgery. But, if the tear is severe enough, surgery may be the only option. In that case, newer techniques are more and more available that are less invasive (read: less cutting!) and that generally means a quicker rehab. See the links below for more detailed stats.
Rehab can be very lengthy and intense depending on how significant the injury is and if there are other issues (labrum or bicep tendon tears for example) to consider. Non-surgical rehab will jump right into exercises in a pain-free range and will first work to decrease pain and then to restore function. Surgical rehab will start primarily with passive range of motion (the therapist moves the arm for you) and then slowly progress with very light exercises. I can't stress enough how important it is for surgical patients to stay within their restrictions and not overdo it. A re-tear is a worst case scenario and entails another surgery with even less potential for rehab. So, mind your surgeon and therapist!
For some stats and pics of surgical repair techniques: http://orthoinfo.aaos.org/topic.cfm?topic=A00406
For video of surgical repairs: http://www.drgartsman.com/rotator_cuff_repair/rotator_cuff_video.asp