There are approximately a zillion websites devoted to Medicare and how it works, but here are some basics:
Medicare is the Federal health insurance program for Americans age 65 and older, and some disabled Americans. The Original Medicare Plan, which is available nationwide, is a fee-for-service plan that is managed by the Federal Government. It pays for many health care services and supplies, but it won't pay all of your health care costs.
Generally, you should enroll in Medicare when you first become eligible. If you choose to enroll at a later time, you will pay a late-enrollment penalty.
If you already have health insurance from an employer or another source, talk to your benefits administrator about whether you should join Medicare or not while still covered.
Medicare has four parts: hospital insurance, known as Part A; medical insurance, known as Part B, which provides payments for doctors and related services; and prescription drug coverage, known as Part D. Medicare Part C gives you the choice of receiving the benefits of Medicare A, B, and D through a private health plan, like an HMO or PPO. This coverage is called Medicare Advantage.
Most people don't pay a premium for Part A, since they already paid for it through payroll taxes while they were working. There is a monthly premium for Medicare Part B.
Usually, you will pay a premium if you decide to enroll in Medicare's prescription drug plan. If you don't enroll as soon as you are eligible, your premium will be higher if you decide to enroll at a later time. Also, once you are past your first eligibility, you will have to wait for the annual enrollment period (generally November 15-December 31 of each year) in order to enroll in Medicare's prescription drug coverage.