A Living Will is a document that sets forth your wishes regarding whether you would like to receive various types of life-sustaining treatments. It takes effect only if you are at the end of your life (e.g., persistent vegetative state or very end stages of dementia). It will not prevent you from being resuscitated if, for example, you are in a car accident and you have the potential to make a recovery.
Creating a Living Will allows you to determine how the end of your life will be handled. It also takes the responsibility (and attendant guilt) for making the incredibly difficult end of life decisions away from your loved ones because you have clearly expressed your desires in advance. They need only let your wishes be carried out.
A copy of your Living Will should also be provided to all of your medical providers to have in their files.