How is osteoarthritis (OA) different from rheumatoid arthritis (RA)?

Arthritis is an umbrella name for more than 100 diseases, including lupus, gout, osteoarthritis (OA), and rheumatoid arthritis (RA), all of which affect a person's joints or connective tissues. But OA and RA are very different diseases, with distinct symptoms and treatments:

Osteoarthritis
When it starts: Later in life, usually after age 50    
Joints affected: Mostly hips, knees, feet, and spine; rarely affects hands           
Joint symptoms: Pain, swelling                                                         
Pain symmetry: No; pain often affects only one joint                               
Bone symptoms: Bony growths

Rheumatoid Arthritis
When it starts: Usually between age 30 and age 60, but can begin anytime
Joints affected: Tends to affect hands and feet first, but can affect any joints
Joint symptoms: Pain, stiffness, heat, redness, tenderness
Pain symmetry: Yes; usually occurs in joints on both sides of the body (e.g., both wrists or both ankles)
Bone symptoms: Erosion of bones in affected joints
Blood test results: Detect inflammation, anemia

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Unlike the more common osteoarthritis, which is mainly a disease of the cartilage in joints, rheumatoid arthritis (RA) occurs when the body's immune system attacks and damages the joints and, sometimes, other organs. RA often occurs in a symmetrical pattern, meaning that if one knee or hand is involved, the other one is too.