Medicare provides critical health benefits to countless seniors, but how much do you really know about the program? Here are a few things you ought to be aware of.

1. It's not totally free

Though most participants don't pay a premium for Medicare Part A, which covers hospital visits, Parts B and D, which cover preventive care and prescription drugs, respectively, will cost you. The standard premium for Medicare Part B is currently $134, though it may be higher depending on your income. Similarly, your Part D costs will depend on your earnings. If you exceed a certain threshold, you'll be subject to an increase on top of your regular Part D premium, as follows:

If You're a Single Filer and Earn...

If You're a Joint Filer and Earn...

You'll Pay...

$85,000 or less

$170,000 or less

Your Part D plan premium alone

above $85,000 up to $107,000

above $170,000 up to $214,000

$13.30 plus your plan premium

above $107,000 up to $160,000

above $214,000 up to $320,000

$34.20 plus your plan premium

above $160,000 up to $214,000

above $320,000 up to $428,000

$55.20 plus your plan premium

above $214,000

above $428,000

$76.20 plus your plan premium

DATA SOURCE: MEDICARE.GOV

2. Enrolling late could cost you

Though you can't use Medicare's services until you turn 65, you can -- and should -- enroll up to three months before the month of your 65th birthday. Signing up for Medicare on time is a good way to keep your premium costs to a minimum. Your initial enrollment period is effective for seven months, beginning three months before the month you turn 65 and ending three months after the month you turn 65. Miss that window, however, and you could see your Part B premiums rise 10% for every year you go without coverage upon becoming eligible. (Note that enrollment is typically automatic if you're already getting Social Security benefits, but it pays to verify that you're signed up.)

Medicare enrollment form

IMAGE SOURCE: GETTY IMAGES.

Being late to the game for Part D could also cost you. If you go 63 days or more without a prescription drug plan, you could face a late enrollment penalty equal to 1% of the national base beneficiary premium -- currently $35.63 -- times the number of full months you fail to obtain coverage. Going 18 months without coverage, for example, will add $6.40 a month to your costs.

3. It won't cover long-term care

Many seniors eventually end up needing assistance with daily living tasks, but if you're counting on Medicare to pick up the tab, think again. While Medicare will cover your immediate health needs, many enrollees are surprised to learn that it doesn't actually cover long-term care expenses, like home health aides that aren't strictly medically necessary. To avoid getting caught off-guard, you might consider purchasing long-term care insurance -- or saving enough to cover what could be some pretty significant out-of-pocket costs.

4. It doesn't provide coverage outside the United States

Many seniors use their newfound free time to travel the world, but if you're embarking on an adventure overseas, be sure to arrange for some supplemental health coverage. If you don't, and you come across a medical issue, it could be costly, because Medicare won't cover you once you exit the country. Because some Medicare Advantage plans offer emergency care outside the United States, it might pay to sign up for one if you expect to travel extensively in retirement.

5. You can receive Medicare services from the comfort of home

Traveling to and from medical appointments can be difficult for some seniors. Thankfully, Medicare's telehealth services offer seniors instant access to medical advice without having to leave their homes. Through this program, you can consult with different providers electronically to get answers to your health questions, thus saving yourself both time and money.

The more you know about Medicare, the better positioned you'll be to maximize your benefits. Even if you're not quite ready to enroll, it still pays to read up on Medicare and find out how it works.