Arthritis is a form of joint disorder that involves inflammation ofone or more joints. There are over 100 different forms of arthritis.The most common form, osteoarthritis (degenerative joint disease), is a result of trauma to the joint, infection of the joint, or age.Other arthritis forms are rheumatoid arthritis, psoriatic arthritis, and related autoimmune diseases.Septic arthritis is caused by joint infection.