Medicare is the U.S. federal health insurance program for individuals who are age 65 or older and people with certain disabilities or end-stage renal disease. It is a major component of the federal government’s safety net for the elderly and handicapped, providing crucial medical coverage for tens of millions of Americans.

Medicare is divided into four categories: Part A, Part B, Part C (known as Medicare Advantage), and Part D. Part A provides coverage for hospital stays and related services. Part B covers outpatient services like doctor visits and exams, preventive care, and certain drugs. Part C is a private health plan with additional services beyond those covered by Part A and Part B. And Part D is a prescription drug program.

Part A is the original version of Medicare, and it's the only part of the program that is free; most people automatically qualify for Part A when they turn 65. Part B and Part C have monthly premiums, and Part D premiums vary depending on the plan you choose. All Medicare plans have annual deductibles, coinsurance, and/or copayments.

Services that are covered by Medicare include hospital stays, doctor visits, preventive care, durable medical equipment, and home health care. Medicare does not cover all medical expenses, though; it does not cover long-term care, cosmetic surgery, or most alternative treatments.

Medicare is funded by a combination of general tax revenues and premiums paid by enrollees. The federal government pays for about three-fourths of the cost of Medicare, while the remaining costs are shared by states, employers, and enrollees.

Medicare has come under fire in recent years for its rising costs and access issues. While the overall quality of care received by Medicare enrollees remains high, efforts have been made to reduce costs and improve access by instituting reforms that focus on prevention and care coordination.

Overall, Medicare is an invaluable part of the social safety net for seniors, providing essential coverage for people who are unable to afford private health insurance or for whom private coverage is not available. Despite its flaws, Medicare is still an important piece of the U.S. health care system that has helped millions of elderly and disabled Americans stay healthy.