Part D is a section of the Medicare program that provides prescription drug coverage to eligible individuals. It was introduced in 2003 to help seniors and people with disabilities afford their medications. Beneficiaries can choose from various private insurance plans that offer different levels of coverage and costs. Enrollment in Part D is optional, but those who do not sign up when first eligible may face late enrollment penalties. The plans typically cover a range of medications, and beneficiaries pay a monthly premium, along with copayments or coinsurance for their prescriptions.