The Medicare program, which is a federal program that helps senior citizens and certain other individuals pay for health care, is divided into four parts. Part A covers hospital and inpatient services and Part B covers doctor visits and other outpatient services. Together, Parts A and B are referred to as Original Medicare. Part C, also known as Medicare Advantage, makes Medicare-covered services available through private health plans, such as HMOs, PPOs, and private fee-for-service plans (PFFSs). Part D prescription drug coverage is offered by private companies through stand-alone plans (for members who have Original Medicare) and through HMOs, PPOs, and PFFSs (for beneficiaries who have Medicare Advantage). Anyone who has Original Medicare or Medicare Advantage is eligible to enroll in Part D. Enrollment in Part D is voluntary.