An advance medical directive is a document in which you can specify the types of life-sustaining medical treatments, pain relief and surgical or other procedures you would want - or would not want - to receive in the case of imminent death from an irreversible condition, or in the case of a persistent vegetative state. It is also sometimes called a personal directive or a living will, but it is not a Will. It is also not a Power of Attorney. For that, you will need a Power of Attorney Form.
An advance directive can speak for you if you can't (for instance, if you're unconscious, in a coma or unable to communicate your wishes concerning medical procedures). It instructs the attending physician whether to administer or withhold pain drugs, blood transfusions, antibiotics, surgery, and other treatments in an effort to extend your life. You can also sign a DNR (Do Not Resuscitate) Order, which removes the burden of making that decision off the shoulders of your loved ones.