Help cartilage. They are the building blocks of normal cartilage. If you can take them in a form that you can absorb thru your GI tract there is some research that shows that 1500 mg + in divided doses can decrease joint pain from arthritic joints. Read book "cure for arthritis" details many of the studies. Studies are on going, vets have been using it for years on dogs with good success.
Rebuild cartilage. The combination of glucosamine and chondroitin has been shown in studies to (1) provide a degree of pain relief in the short term, and (2) trigger growth of new cartilage when taken for at least two years straight. So it can help you pain and reverse some of the chronic damage. You can find this in the health food store. MSM is not necessary and adds to the cost of the product.