Arthritis literally means inflammation of a joint. It is more commonly known as wearing out of our joints. When arthritis occurs, cartilage (which is the material that covers the ends of the bones) begins to wear leading to inflammation and pain.
When enough cartilage wears off then bone will be exposed and lead to persistent pain and disability. There are many different types of arthritis.
These can include Osteoarthritis and inflammatory types such as gout and rheumatoid arthritis. In these types the joints are inflamed because of crystals or the body attacking its own tissues. Osteoarthritis is the "wear and tear" arthritis and is the most common type. This is the arthritis that usually affects older individuals and is most commonly seen in the hands, knees and hips.
Above is an x-ray showing arthritis of the right knee with loss of cartilage and bone on bone contact.