Most of us know there’s Medicare Part A, Part B, Part D, Medigap plans, Medicare Advantage plans but don’t really know the details, similarities and differences all that well.

We know Medicare is an extremely important component of retirement, but when is the enrollment period? What are the requirements for enrollment? Income thresholds for higher coverage? And what exactly is the donut hole?

To help you understand this intricate health insurance program, we’ve answered the who, what, when, where and why and made a list of 11 basic things you absolutely need to know about Medicare.