Medicare is a national health insurance program for people 65 years of age or older and under 65 for those receiving Social Security disability benefits.  Participation is, for the most part, mandatory if you want health insurance in this country.  And while you can have additional private insurance you must enroll in Medicare in order for that supplemental insurance to take effect.

Once you turn 65, Medicare becomes the primary payer and under original Medicare, any private insurance you have is secondary and will not pay until Medicare has paid its part.  The only insurance that remains primary to Medicare is an employer group plan that covers 20 or more employees. 

The articles below provide a variety of information covering different areas related to Medicare.