Short answer: Part A covers hospital/inpatient care, Part B covers doctors and outpatient care, Part C (Medicare Advantage) is a private all-in-one alternative to A and B, and Part D covers prescription drugs.
Medicare has four parts. Part A (hospital insurance) covers inpatient hospital stays, skilled nursing, and hospice, usually premium-free if you or a spouse paid Medicare taxes long enough. Part B (medical insurance) covers physician visits, outpatient care, and preventive services for a monthly premium. Part C (Medicare Advantage) is private coverage that bundles A and B (and usually D) into one plan, often with extra benefits and a network. Part D covers prescription drugs through private plans. People generally choose either Original Medicare (A + B, often with a Part D plan and a Medigap policy) or a Medicare Advantage plan.