Medicare is a national social insurance program administered by the U.S. government that provides health insurance coverage for individuals aged 65 and older, individuals under 65 with disabilities, and individuals with end-stage renal disease. It has four parts - Part A covers inpatient care, Part B covers outpatient care, Part C are private Medicare Advantage plans that combine Parts A, B, and sometimes D, and Part D is prescription drug coverage. Individuals can enroll in Medicare during an initial enrollment period or special enrollment periods. Premiums and costs vary depending on income and type of coverage selected.