The coverage provided by Medicare Part A and Part B together forms what is known as traditional or original Medicare — so named because that was the extent of the program's coverage when it began back in 1966. It's also called fee-for-service Medicare because each provider — whether it's a doctor, hospital, laboratory, medical equipment supplier, or whatever — is paid a fee for each service.
But these days Medicare also offers an alternative to the traditional program: a range of health plans that mainly provide managed care through health maintenance organizations (HMOs) or preferred provider organizations (PPOs). These plans are run by private insurance companies, which decide each year whether to stay in the program. Medicare pays each plan a fixed fee for everyone who joins that plan, regardless of how much or little health care a person actually uses. This health plan program is called Medicare Advantage or Medicare Part C.
Medicare Advantage plans must, by law, cover exactly the same services under Part A and Part B as traditional Medicare does. (So if you need knee replacement, for example, the procedure is covered — regardless of whether you're enrolled in a Medicare Advantage plan or in the traditional program.) But the plans may also offer extra benefits that traditional Medicare doesn't cover — such as routine vision, hearing, and dental care. Most plans include Part D prescription drug coverage as part of their benefits package.
Still, being enrolled in one of these plans is a very different experience from using the traditional Medicare program. Your out-of-pocket costs are different, and so are your choices of doctors and other providers.