Medicare is a federally funded program that provides medical care to senior citizens 65+ years old. Medicare is also available to disability beneficiaries who receive Disability Insurance Benefits (DIB) through Social Security. Senior citizens are always guaranteed to be covered, but Medicare isn’t entirely free–patients are often required to pay part of their medical costs. Since Medicare is run by the federal government, coverage is basically the same universally throughout the United States.
Medicaid provides financial assistance to low income individuals of any age, as demonstrated need presents itself. Your income must be below a certain amount in order to receive Medicaid. Patients are sometimes required to pay nothing, but there is sometimes a co-pay. Medicaid is run jointly by both the states and the federal government so rules and regulations vary depending on your state of residence.
It is possible to be on both Medicare and Medicaid, depending on eligibility for both programs.