The term arthritis refers to about 120 different diseases that can affect the joints, muscles and other soft tissues. The three most common forms are osteoarthritis, fibromyalgia and rheumatoid arthritis.steoarthritis, also known as degenerative joint disease, is the most common type of arthritis, affecting an estimated 21 million adults in this country. Commonly referred to as a wear and tear arthritis, osteoarthritis involves destruction of the cartilage, the cushion or shock absorber on the ends of the bones.