Internist (not to be confused with intern -- not at all the same thing)
Specialist in internal medicine; looks at all internal organs (competent in cardiology, endochrinology, etc.).
They can diagnose problems with:
Allergy and immunology (treatment of immune system disorders)
Cardiology (treatment of heart disorders)
Endocrinology (treatment of hormonal disorders)
Gastroenterology (treatment of digestive system disorders)
Hematology (treatment of blood and bone marrow disorders)
Nephrology (treatment of kidney disorders)
Medical Oncology (treatment of cancer with medicine [e.g., chemotherapy])
Pulmonology (treatment of lung disorders)
Rheumatology (treatment of musculoskeletal disorders)
Not all internist do ALL of the above, but they will do most.
They keep studying, on average, 4 years AFTER becoming MD and some will even spend another year or two specializing even more in one or two things on the list.
They can diagnose the problem but they usually don't operate, they leave that to an appropriate specialist surgeon, once they have identified the problem.