Medicare Supplement Insurance, or Medigap, is health insurance coverage provided by private companies designed to cover excess costs not covered by original Medicare.

For more info- https://www.investopedia.com/terms/m/medigap-insurance.asp

Medicare Advantage plans, also referred to as Part C plans, are part of the Medicare program for senior citizens and disabled adults who qualify. Private companies provide Medicare Advantage plans instead of the federal government, and these plans typically include the same Part A hospital, Part B medical coverage and Part D drug coverage that Medicare does, with the exception of hospice care. As of 2017, about one third of the 57 million Medicare participants were enrolled in a Medicare Advantage plan.

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Medicare Part D is a prescription drug benefit program that was created through the U.S. Medicare Prescription Drug, Improvement, and Modernization Act of 2003. This law is sometimes also called the Medicare Modernization Act, or MMA.

The program gives Medicare recipients these basic choices: stay in traditional Medicare without signing up for the prescription drug benefit outlined in the Act, stay in traditional Medicare and enroll in a Medicare drug plan, enroll in other Medicare plans, or enroll in a comprehensive private health plan, which may or may not cover prescription costs. The program began providing coverage for users on January 1, 2006.

For more info- https://www.investopedia.com/terms/m/medicarepartd.asp

Official Medicare site:  https://www.medicare.gov/

CMS- https://www.cms.gov/