Osteoarthritis, the most common, affects about 27 million Americans. Though formerly considered a disease that's exclusive to seniors, doctors now realize that it can affect people at any age. Rheumatoid arthritis affects about 1.3 million Americans regardless of age group.
Osteoarthitis is loss of cartilage around joints that results in bone rubbing painfully against bone, weakening surrounding bones. Cartilage is the cushion between bones so, if your cartilage is worn down or torn as a result of injury, you don't have a cushion between bones to ease joint movement.
Rheumatoid arthritis reveals itself in stiffness, swelling, weakness, loss of mobility and eventual destruction of joints. It can flare up, disappear, then flare up again, giving you "good" days and "bad" days. The cause of rheumatoid arthritis is still unknown, but once you have it, it's progressive.