The Arthritis Foundation classifies the more than 100 types of arthritis into four categories: degenerative, inflammatory, infectious, and metabolic. Degenerative arthritis includes osteoarthritis, which is the most common form of arthritis. Inflammatory arthritis includes rheumatoid arthritis, another of the most common forms. Metabolic arthritis includes gout, among other conditions, while infectious arthritis can be caused by bacteria or a fungus or virus.