"Medicare is a federal program that pays for covered health care services of qualified beneficiaries. It was established in 1965 under Title XVIII of the Social Security Act to provide health insurance to individuals 65 and older, and has been expanded over the years to include permanently disabled individuals under the age of 65. Medicare, which consists of four parts (A-D), covers hospitalizations, physician services, prescription drugs, skilled nursing facility care, home health visits, and hospice care, among other services. Generally, individuals are eligible for Medicare if they or their spouse worked for at least 40 quarters in Medicare-covered employment, are 65 years old, and are a citizen or permanent resident of the United States. Individuals may also qualify for coverage if they are a younger person who cannot work because they have a medical condition that is expected to last at least one year or result in death, have end-stage renal disease (permanent kidney failure requiring dialysis or transplant), or have amyotrophic lateral sclerosis (ALS, Lou Gehrig's disease). The program is administered by the Centers for Medicare & Medicaid Services (CMS), and by private entities that contract with CMS to provide claims processing, auditing, and quality oversight services."
CRS Report for Congress, R40425
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html