There are several types. Osteoarthritis is the "wear and tear" type. Rheumatoid arthritis is an autoimmune disorder. The body actually attacks the joints and deforms bone and cartridge causing pain and other problems as well. Fibromyalgia is a soft tissue arthritis that causes widespread pain and fatigue. Here is a link to more info
Arthritis is a part of the aging process as the cartilage wears out in our joints a bit. People can have arthritis at any age, though, especially athletes who stress their joints a lot. Rheumatoid arthritis can develop in anyone and attacks the joints.