Part A
In general, Part A covers:

• Hospital care
• Skilled nursing facility care
• Nursing home care (as long as custodial care isn't the only care you need)
• Hospice
• Home health services

Part B
Part B covers 2 types of services:

• Medically necessary services: Services or supplies that are needed to diagnose or treat your medical condition and that meet accepted standards of medical practice. Preventive services: Health care to prevent illness (like the flu) or detect it at an early stage, when treatment is most likely to work best.
• You pay nothing for most preventive services if you get the services from a health care provider who accepts assignment.

Part B covers things like:

• Clinical research 
• Ambulance services
• Durable medical equipment (DME)
• Mental health
• Inpatient
• Outpatient
• Partial hospitalization
• Getting a second opinion before surgery
• Limited outpatient prescription drugs

Part D

• Each Medicare drug plan has its own list of covered drugs (called a formulary). Many Medicare drug plans place drugs into different "tiers" on their formularies. Drugs in each tier have a different cost.
• A drug in a lower tier will generally cost you less than a drug in a higher tier. Your prescriber may think you need a drug that's on a higher tier. If so, you or your prescriber can sometimes ask your plan for an exception to get a lower copayment.
• A Medicare drug plan can make some changes to its formulary during the year within guidelines set by Medicare. If the change involves a drug you’re currently taking, your plan must do one of these:
• Provide written notice to you at least 60 days prior to the date the change becomes effective.
• At the time you request a refill, provide both of these:
• A written notice of the change
• A 60-day supply of the drug under the same plan rules as before the change