Medicare Prescription Drug Plans are offered by insurance companies and other private companies approved by Medicare. They add coverage to:

Original Medicare
Some Medicare Cost Plans
Some Medicare Private Fee-for-Service Plans and
Medicare Medical Savings Account Plans
With a Medicare Prescription Drug Plan:

Generally, you pay less for your prescriptions.
You will get a plan member card after you enroll. You use this card when you go to the pharmacy to get your prescriptions filled.
You will pay the co-payment, coinsurance, and/or deductible, if any
If you have limited income and resources, you may get extra help to pay for your Medicare drug plan costs.If you want to compare Medicare Prescription Drug Plans, use the Medicare Prescription Drug Plan Finder.