Medicare's four "parts" aren't tiers of one plan — they're separate pieces of coverage that combine in different ways depending on how you choose to get your Medicare. * Part A is hospital insurance: inpatient hospital stays, skilled nursing facility care, hospice, and some home health care. Most people don't pay a monthly premium for Part A at all, since it's covered by Medicare payroll taxes you already paid during your working years, as long as you worked and paid into the system for at least 10 years. * Part B is medical insurance: doctor visits, outpatient care, preventive services, durable medical equipment, and other services outside a hospital stay. Unlike Part A, most people do pay a monthly premium for Part B, and that premium amount is tied to your income. * Together, Part A and Part B make up what's called "Original Medicare." Part C, known as Medicare Advantage, is an alternative way to get that same coverage: instead of Original Medicare directly from the government, you enroll in a plan sold by a private insurer that's required to cover at least everything Original Medicare covers, often bundling in prescription drug coverage and extra benefits like dental or vision. * Part D is prescription drug coverage, and it's separate from everything else — you add it either as a standalone plan alongside Original Medicare, or it may already be built into your Medicare Advantage plan. If you have Original Medicare without a Part D plan and don't have other qualifying drug coverage, you can face a lasting late-enrollment penalty for signing up later.