Part A covers hospitalization. This typically does not have a premium. If you were to be hospitalized, you would have a deductible for the first 60 days of inpatient hospitalization. The deductible amount varies from year to year.
Part B covers medical or outpatient services. You will have a premium for this and generally there is an annual deductible and 20% co-insurance. There is not a maximum cap that you could be responsible to pay out of pocket.
Part C, also called Medicare Advantage plans, come in the forms of HMO, PPO or PFFS. They typically combine your parts A, B and D into one plan. These are through private insurance companies that generally offer extra benefits. The monthly premiums start at $0/month. Each insurance company offers different co-pays and deductibles.
Part D is your Drug coverage. These are also through private insurance companies. Part D prescription plans can be combined in an Advantage plan or be stand alone. This coverage should be purchased at time of eligibility to avoid penalties down the road.