A living will is not actually a will in the same way you'd think of one in the case of someone passing away. A typical will distributes property and sets up guardianship in some cases; it's distributed after a person's passing and typically following a funeral. This kind of will may also include an estate plan for how the person's property will be cared for after his passing.
In contrast, living wills don't require someone to have passed away to be put to use. Living wills are also known as health care directives and/or directives to physicians. These wills express your desires if you're unable to communicate your preferences when it comes to medical treatment.
For example, your living will may state that you don't want to be placed on life support and would prefer a natural death if you can't breathe or function on your own. If you're older, you may say you don't want artificial or advanced medical techniques used if you suffer a heart attack or other life-threatening illness.
These documents are recognized by law and are enforceable. They may have information about your preferences on tube feeding, resuscitation and organ donation in the case of death. As long as the living will is valid, the health care provider is bound by the document.
While many living wills restrict medical care, some can encourage the use of all necessary and possible treatments. Sometimes, medical options can be difficult to explain and many treatments may be available; a patient can use a living will to directly state what is or is not acceptable in his or her care.
Source: FindLaw, "Living Wills: Introduction," accessed May 06, 2016