Living wills and other advance directives are written, legal instructions regarding your preferences for medical care if you are unable to make decisions for yourself. Advance directives guide choices for doctors and caregivers if you're terminally ill, seriously injured, in a coma, in the late stages of dementia or near the end of life. (http://www.mayoclinic.org/healthy-living/consumer-health/in-depth/living-wills/art-20046303)
I cannot stress enough how important it is to have a written living will and designate someone to be your durable power of attorney (also called health care proxy) to make medical decisions for you if you are unable to do so! It doesn't matter if you're a young college whippersnapper with the world ahead of you (I'm talking to you Kristen!), or an old lady like me (LOL). These are our bodies, and we should be the ones to decide which medical treatments we do and do not want to be subjected to.
The advantage of having these documents is that they clearly state your wishes when you are not able to speak for yourself. Of course, you'll need to discuss the documents with your health care provider and loved ones ahead of time so that everyone is on the same page, and make sure they have a copy. If you do that, your loved ones can avoid a lot of anxiety and confusion if you are unconscious and in a coma. Remember Terri Schiavo? There was a lot of speculation about what Terri would have wanted, but no written instructions.
As some of you know, I'm a paralegal, so now is the time for me to tell you to go hire a lawyer to assist with these documents. Good advice, sure, but if you can't or won't use a lawyer, you're in luck! You can easily find tons of information online and even get forms that are specific to your state. Google is your friend, or try here: http://uslwr.com/formslist.shtm
You will find that the forms are not that difficult to fill out. So do yourself a huge favor, take an hour or so to get your forms, fill them out, discuss with your family, and then fax them over to your healthcare provider. You just never know......