A Medigap policy (also called "Medicare Supplement Insurance") is private health insurance that is designed to supplement Original Medicare. This means it helps pay some of the health care costs ("gaps") that Original Medicare doesn’t cover (like copayments, coinsurance, and deductibles). If you have Original Medicare and a Medigap policy, Medicare will pay its share of the Medicare-approved amounts for covered health care costs. Then your Medigap policy pays its share. A Medigap policy is different from a Medicare Advantage Plan (like an HMO or PPO) because those plans are ways to get Medicare benefits, while a Medigap policy only supplements your Original Medicare benefits.

Every Medigap policy must follow Federal and state laws designed to protect you, and the policy must be clearly identified as "Medicare Supplement Insurance." Medigap insurance companies in most states can only sell you a "standardized" Medigap policy identified by letters A through N. Each standardized Medigap policy must offer the same basic benefits, no matter which insurance company sells it. Cost is usually the only difference between Medigap policies with the same letter sold by different insurance companies.

Source:  www.medicare.gov