Medicare is divided into multiple letters that represent different Medicare plans. The main plans that you hear about are Medicare part A, B, C, &D.
Medicare part A covers hospital expenses.
Medicare part B covers doctor office visits
Medicare part C, is actually a managed care replacement plan for Medicare Parts A, B, & D and are offered through private companies. If you have a Medicare Part C plan, you would give your doctor your health insurance card and not your red, white, and blue, Medicare card.
Medicare Part D is prescription coverage.
If you have Medicare Part A and/or B then you may also purchase Medigap, supplemental insurance to help cover costs that Medicare does not cover. Medigap insurance may also help pay deductibles, co-pays, and or co-insurance.
This post is meant as a very high level overview; details will be available in further posts or in tutorials/eBooks, etc available for purchase.