Medicare is a federally funded health insurance program for individuals age 65 and older which aims to provide quality health care to seniors.  Individuals may join 3 months before or after reaching age 65. 

There are four parts or options that cover specific medical services through Medicare, Parts A through D.  These are explained here.

Since Medicare does not cover all medical expenses due to "gaps" between the policy options, private insurance companies offer Medigap (Medicare Supplemental Insurance) plans to cover areas that Medicare does not pay.