Medicare is a federal health insurance program for people aged 65 and older, younger people with disabilities, and people with end-stage renal disease. It has four parts: Part A covers hospital insurance; Part B covers medical insurance; Part C are private Medicare Advantage plans; Part D covers prescription drug plans. People can enroll in Medicare during an initial enrollment period around their 65th birthday or a general enrollment period each year and may have to pay premium penalties if they delay enrollment. Premiums and deductibles apply to Parts A and B.