Medicare is a health insurance program provided by United States of America to people over 65 and over. It is administered by CMS (Center for Medicare and Medicaid Services).There are 4 parts in Medicare:

Part A for Hospital Insurance
Part B for Medical Insurance
Part C for Medicare Advantage
Part D for Prescription Drug Coverage

Following websites have very useful information about Medicare,
Wikipedia
Social Security Admninistration
Center for Medicare

Below is a video describing the fundamentals of Medicare