Types of Arthritis

Primary forms of arthritis:
    * Osteoarthritis
    * Rheumatoid arthritis
    * Septic arthritis
    * Gout and pseudo-gout
    * Juvenile idiopathic arthritis
    * Still's disease
    * Ankylosing spondylitis

Secondary to other diseases:
    * Ehlers-Danlos Syndrome
    * Sarcoidosis
    * Henoch-Schönlein purpura
    * Psoriatic arthritis
    * Reactive arthritis
    * Haemochromatosis
    * Hepatitis
    * Wegener's granulomatosis (and many other vasculitis syndromes)
    * Lyme disease
    * Familial Mediterranean fever
    * Hyperimmunoglobulinemia D with recurrent fever
    * TNF receptor associated periodic syndrome
    * Inflammatory bowel disease (Including Crohn's Disease and Ulcerative Colitis)

Diseases that can mimic arthritis include:
    * Hypertrophic osteoarthropathy
    * Multiple myeloma
    * Osteoporosis
    * Fifth disease (parvovirus infection)

Source: http://en.wikipedia.org/wiki/Arthritis