A rheumatologist is an internist with advanced training in diseases of the joints, muscles and tendons. Rheumatologists treat rheumatoid arthritis, osteoarthritis and gout, for example. They also treat autoimmune conditions like lupus and scleroderma.
An internal medicine physician or internist diagnoses and treats adults for a wide variety of conditions. These range from minor disorders like allergies to complex conditions like diabetes and heart disease.