An advance medical directive is a document in which you can specify the types of life-sustaining medical treatments and procedures you want or do 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). You can direct medical personnel to administer or withhold pain drugs, blood, antibiotics, etc. You can also sign a DNR (Do Not Resuscitate) Order, which removes the burden of making such a decision from your loved ones.