Medicare is a national health insurance program in the United States that provides coverage to people who are 65 or older, as well as to some younger people with disabilities or end-stage renal disease (ESRD). It is administered by the Centers for Medicare and Medicaid Services (CMS), a federal agency within the Department of Health and Human Services (HHS).

There are four parts to Medicare:

• Part A: Hospital insurance that covers inpatient care in hospitals, skilled nursing facilities, and hospice care.
• Part B: Medical insurance that covers doctors' services, outpatient care, medical supplies, and preventive services.
• Part C: Medicare Advantage plans, which are private insurance plans that are approved by Medicare and provide all the benefits of Parts A and B, as well as additional coverage, such as prescription drugs and routine vision and dental care.
• Part D: Prescription drug coverage, which is offered through private insurance plans that are approved by Medicare.

People who are eligible for Medicare can enroll in one or more of these parts, depending on their needs.