Medicare Supplement (Medigap) insurance is designed to fill "gaps" in costs that Medicare Parts A and B do not cover. Medicare Supplement generally covers the remaining balance left after Medicare has paid its share, depending on the level of coverage you select.
Medigap does not include prescription drug coverage, so you should consider enrolling in a Prescription Drug plan (Part D) as well.
While there are a number of different Medigap options offered by private insurance companies on the Aon Retiree Health Exchange, benefits are standardized in each state. This means that the coverage of each plan offered by one insurance company will be the same as the coverage for the same plan offered by any other insurance provider. The difference among the Medigap plans within your state lies in the details of how much coverage is offered and which gaps will be filled.
Medicare Supplement Plans make sense if you want flexibility in choosing your doctors, visit your doctors frequently, or travel extensively.