A Health Care Power of Attorney is a document that allows you to designate a person (an "Agent") who will have the authority to make health care decisions on your behalf if you are unconscious, mentally incompetent, or otherwise unable to make such decisions. In many states you can also express your wishes regarding whether you wish to receive "life-sustaining procedures" if you become permanently comatose or terminally ill, in the Health Care Power of Attorney document. This will help your agent to know your wishes as he or she makes decisions for you. Even if you do include this in the document, you should still discuss the Health Care Power of Attorney with the Agent, expressing your wishes, values and preferences regarding health care.
A Health Care Power of Attorney is different from a Living Will because it allows you to appoint someone to make health care decisions for you. A Living Will only allows you to express your wishes concerning life-sustaining procedures.