In every joint, the bone ends are covered by a tough yet slippery joint surface called articular cartilage. Arthritis is the wear and tear of these joint surfaces leading to pain, stiffness, deformity and disability. 

The most common cause of arthritis is due to wear and tear (osteoarthritis). However, other conditions like rheumatoid disease, psoriasis, gout etc. can also cause severe inflammation and damage the joint surface (inflammatory arthritis), resulting in similar or worse symptoms.

The two x-ray examples shown below clearly demonstrate the difference between a healthy joint with good joint space and an arthritic joint with complete loss of the joint space, resulting in bone on bone arthritis (severe arthritis).