As noted by the Centers for Medicare and Medicaid Services (CMS):

"Medicare is health insurance for people 65 or older, people under 65 with certain disabilities, people of any age with End-‑Stage Renal Disease (ESRD) (permanent kidney failure requiring dialysis or a kidney transplant).  The different parts of Medicare help cover specific services:

• Medicare Part A (Hospital Insurance) helps cover inpatient care in hospitals, skilled nursing facility, hospice, and home health care.
  
  
• Medicare Part B (Medical Insurance) helps cover services from doctors and other health care providers, hospital outpatient care, durable medical equipment, and home health care.  Medicare Part B also includes preventive services to help maintain your health and to keep certain illnesses from getting worse.
  
  
• Medicare Part C (Medicare Advantage) includes all benefits and services covered under Part A and Part B.  [Medicare Advantage plans are] run by Medicare-approved private insurance companies and usually includes Medicare prescription drug coverage (Part D) as part of the plan and may include extra benefits and services for an extra cost (MAPD).
  
  
• Medicare Part D (Medicare Prescription Drug Coverage) helps cover the cost of outpatient prescription drugs and are run by Medicare-approved private insurance companies.  [Medicare Part D plans] may help lower your prescription drug costs and help protect against higher costs in the future.
  

And a more formal explanation with reference to Medicare funding

"[Medicare is a] nationwide, federally administered health insurance program authorized in 1965 under Title XVIII of the Social Security Act to cover the cost of hospitalization, medical care, and some related services for most people age 65 and over.

In 1972, lawmakers extended [Medicare] coverage to people receiving Social Security Disability Insurance payments for 2 years and people with end-stage renal disease. (For beneficiaries whose primary or secondary diagnosis is Amyotrophic Lateral Sclerosis, the 2-year waiting period is waived.)

In 2010, people exposed to environmental health hazards within areas under a corresponding emergency declaration became Medicare-eligible.

In 2006, prescription drug coverage was added as well.

Medicare consists of two separate but coordinated trust funds: Hospital Insurance (HI, or [Medicare] Part A) and Supplementary Medical Insurance (SMI). The SMI trust fund comprises two separate accounts: the [Medicare] Part B account and the [Medicare] Part D account.

Almost all persons who are aged 65 and over or disabled and who are entitled to HI [Medicare Part A] are eligible to enroll in Part B and Part D on a voluntary basis by paying monthly premiums."

2017 ANNUAL REPORT OF THE BOARDS OF TRUSTEES OF THE FEDERAL HOSPITAL INSURANCE AND FEDERAL SUPPLEMENTARY MEDICAL INSURANCE TRUST FUNDS From THE BOARDS OF TRUSTEES, FEDERAL HOSPITAL INSURANCE AND FEDERAL SUPPLEMENTARY MEDICAL INSURANCE TRUST FUNDS

(see:  https://www.cms.gov/ Research-Statistics-Data-and-Systems/ Statistics-Trends-and-Reports/ ReportsTrustFunds/ Downloads/TR2017.pdf)