Medicare Advantage Plans are private insurance plans contracted by the Government. They provide Part A (hospital) Part B (medical) coverage, and some plans include Part D (Rx) coverage.
Medicare Supplemental Insurance Plans are health insurance plans designed to supplement Original Medicare. They help pay some of the health care costs that Original Medicare doesn't cover (including copayments, coinsurance, and deductibles).
Part D (Medicare prescription drug benefit) is an optional US federal-government program to help Medicare beneficiaries pay for self-administered prescription drugs through prescription drug insurance premiums.