Medicare has four parts: Part A, B, C and D and one additional type of plan called Medicare Supplement (Medigap).
Sometimes referred to as hospital insurance, Part A helps pay for overnight hospital stays including your room, tests, and doctor fees. For most people (i.e. as long as you have paid into social security for 10 years), when they turn 65, they are enrolled into Part A automatically and there is no cost. All the other Medicare Parts have costs.