Medicare Part D is a voluntary outpatient prescription drug benefit for people with Medicare provided through private plans that contract with the federal government. Beneficiaries can choose to enroll in either a stand-alone prescription drug plan (PDP) to supplement traditional Medicare or a Medicare Advantage plan, mainly HMOs and PPOs, that provides all Medicare-covered benefits, including prescription drugs (MA-PD).
(Source:https://www.kff.org/medicare/fact-sheet/an-overview-of-the-medicare-part-d-prescription-drug-benefit/)