Medicare has four parts that work together to provide necessary medical coverage. U.S. citizens become eligible for all Medicare parts when they turn 65, or earlier if they have a disability.

Part A

Part A covers hospital expenses while a patient is sick or injured. It's funded by the Medicare taxes the patient paid while he or she was working. Everyone is automatically enrolled in Part A when they turn 65, but may opt out if they choose a private insurance plan. The government is responsible for administering this part of Medicare.

Part B

Part B covers basic routine care, much like a regular health insurance plan. However, unlike Part A, patients must enroll in Part B if they desire coverage. In addition, payments for Part B insurance are automatically deducted from participants' social security checks. Part B is also administered by the government.

Part C

Coverage under Parts C and D replace coverage under Parts A and B. Part C is administered by private insurance companies, who also decide what coverage to offer participants. The private insurance companies work with the government, doctors, and patients to make medical decisions, as well as consolidate patients' medical information and payments. Participants pay for Part C out of pocket, so if you're considering Part C insurance, be sure to compare different policies to find the best deal.

Part D

Part D covers prescription medication that is not otherwise covered under Part C. Like Part C, Part D is managed by private insurance companies and paid for out of pocket.