A Medicare Supplement Insurance (Medigap) policy, sold by private companies, can help pay some of the health care costs that Original Medicare doesn't cover, like copayments, coinsurance, and deductibles.
Medicare Advantage Plans (like an HMO or PPO) also called “Part C” are health plans run by Medicare-approved private insurance companies. Medicare Advantage Plans include Part A, Part B, and usually other coverage like Medicare prescription drug coverage (Part D), sometimes for an extra cost.
Medicare Special Needs Plans are a type of Medicare Advantage Plan designed for certain types of people with Medicare. Some Special Needs Plans are for people with certain chronic diseases and conditions, some are for people who have both Medicare and Medicaid, and some are for people who live in an institution such as a nursing home.
Part D (Medicare Prescription Drug Coverage) helps cover cost of prescription drugs, may lower your costs and protect against higher costs.
Information is from: https://www.medicare.gov/