If you have Medicare Parts A and B, you can join a Medicare Advantage plan. Under Medicare Advantage beneficiaries may choose from a number of private health plans, which include the following:

• Health maintenance organization (HMO) plans that require you to go to doctors, other health care providers, or hospitals on the plan's list, except in an emergency. You may also need to get a referral from your primary care doctor to see a specialist.
• Preferred provider organizations (PPOs) that allow the use of doctors and hospitals outside the plan network for an extra out-of-pocket cost.
• Private fee-for-service plans (PFFSs), which are a Medicare-approved private health insurance plan for which Medicare pays part of the cost. The plan determines how much it will pay doctors, other health care providers, and hospitals, and how much you must pay when you get care.  
• Special needs plans (SNPs), which limit membership to people with specific diseases or characteristics. The plan tailors benefits, provider choices, and drug formularies to best meet the specific needs of the groups they serve.
• Medical savings account (MSA) plans, which allow you to join a high-deductible plan and also set up a special medical savings account with a bank the plan selects. Medicare then gives the plan an amount of money each year for your health care, and the plan deposits some money into your account. You can use the money in your account to pay your health care costs, including health care costs that aren’t covered by Medicare. Money left in your account at the end of the year stays in the account and may be used for health care costs in future years.

Once you choose a plan, you can only change plans during certain times of the year. You can switch plans during the open enrollment period which runs from October 15 through December 7 every year. You can also join a Medicare drug plan during that period. There is also a disenrollment period during which you can switch from Medicare Advantage to original Medicare. That period runs from January 1 through February 14. Beneficiaries who are happy with the way they are receiving Medicare can stay with that program, unless the program stops participating in Medicare. New Medicare enrollees who do not choose a particular program will automatically be enrolled in traditional Medicare.