Living Wills and Advance Directives
A living will (or "medical directive" or "declaration" or "directive to physicians") is simply a written instruction spelling out any treatments you want or don’t want if you are unable to speak for yourself and terminally ill or permanently unconscious. A living will says in effect, "Whoever is deciding, please follow these instructions!" On its own, a living will is very limited — it usually applies only to end-of-life decisions, and standard instructions tend to be general. Unless you have a good crystal ball, it is impossible to anticipate every future medical scenario.
Health care power of attorney
A health care power of attorney (or health care "proxy," or "medical power of attorney") is a document that appoints someone of your choosing to be your authorized "agent" (or "attorney-in-fact" or "proxy"). You can give your agent as much or as little authority as you wish to make health care decisions. The decisions are not limited to just end-of-life decisions. Appointing an agent provides someone with authority to weigh all the medical facts and circumstances and interpret your wishes accordingly. A health care power of attorney is broader and more flexible than the living will.
Health care advance directive
A comprehensive health care advance directive combines the living will and the health care power of attorney into one document. In addition, you may include any other directions, including organ donation or where and how you prefer to be cared for. Because it is more comprehensive and more flexible than the other tools, it is the preferred legal tool.
Source: American Bar Association, www.abanet.org/publiced/practical/directive_livingwill.html.