A means to make health care wishes known in advance to family, loved ones and health care professionals. 

Ideally this should be memorialized in a legal document called a Living Will. 

One may appoint a person to make health care decisions  in the event one is unable to make decisions for ones self.  This person is called the Health Care Agent.  The form that this is documented on is a Health Care Proxy Form.

Advance Directives are not only about “Pulling the Plug.

Even if one is not able to make health care decisions even temporarily in the case of an accident or one is disoriented, or one is having surgery someone else must make decisions during this time.  Appointing an agent allows one to remain in control of the medical treatment that will be received.  Advance directives allow one to communicate wishes to a loved one and health care professionals so that all may act on in accordance with this person's wishes.