"Medicare supplement policies are health insurance policies sold by private insurance companies to fill gaps in Original Medicare."
"A Medicare Advantage Plan (like an HMO or PPO) is another Medicare health plan choice you may have as part of Medicare. Medicare Advantage Plans, sometimes called “Part C” or “MA Plans,” are offered by private companies approved by Medicare."
Medicare Part D (Prescription Drugs)
Medicare Part D - "Medicare offers prescription drug coverage to everyone with Medicare. If you decide not to join a Medicare drug plan when you’re first eligible, and you don’t have other creditable prescription drug coverage, or you don’t get Extra Help , you’ll likely pay a late enrollment penalty ."