While often referred to as if it were a single disease, arthritis is actually an umbrella term used for a group of more than 100 medical conditions that collectively affect nearly 46 million adults and 300,000 children in America alone. While the most common form of arthritis - osteoarthritis (OA) - is most prevalent in people over 60, arthritis in its various forms can start as early as infancy. Some forms affect people in their young-adult years as they are beginning careers and families and still others start during the peak career and child-rearing years.

The common thread  among these 100-plus conditions  is that they all affect the musculoskeletal system and specifically the joints - where two or more bones meet. Arthritis-related joint problems include pain, stiffness, inflammation and damage to joint cartilage (the tough, smooth tissue that covers the ends of the bones, enabling them to glide against one another) and surrounding structures. Such damage can lead to joint weakness, instability and visible deformities that, depending on the location of joint involvement, can interfere with the most basic daily tasks such as walking, climbing stairs, using a computer keyboard, cutting your food or brushing your teeth.

For many people with arthritis, however, joint involvement is not the extent of the problem. Many forms of arthritis are classified as systemic, meaning they can affect the whole body. In these diseases, arthritis can cause damage to virtually any bodily organ or system, including the heart, lungs, kidneys, blood vessels and skin. Arthritis-related conditions primarily affect the muscles and the bones. what_is_arthritis link

More than 100 forms of arthritis and related diseases exist affecting approximately 46 million Americans today.
Source: http://www.arthritis.org