If you choose to stay with original Medicare, you may purchase a supplement to help pay the costs that Medicare doesn't cover such as coinsurance and deductibles.  If you need prescription coverage, you may purchase a seperate Part D plan.  Under this scenario, you will have 3 different monthly premiums to pay: your Part B premium, your supplement premium, and your prescription plan premium.  You may also be required to file claims for service to all three different plans.

If you choose an Advantage plan (Part C), all of the above choices are combined with one insurance company.  Some refer to Advantage Plans as "all in one" plans.  There is a cost savings to these plans also, in fact many companies have plans with $0 premiums.

NOTE:  It is illegal for someone to sell you a supplement plan if you are enrolled in an Advantage plan.  If you purchase a supplement plan while enrolled in Advantage, you will automatically be dropped from the Advantage plan.  If you are in an Advantage plan and want to switch to a supplement, you must disenroll from the Advantage plan first during the disenrollment period.  (see important dates section)