An advance directive is a legal document, completed and signed by you, stating your healthcare treatment and care choices, your living will, and usually names your healthcare power of attorney (HPOA)--the person or people you choose to make healthcare decisions on your behalf if you're unable to speak for yourself.
Your legal HPOA, which can be a separate document from your living will, assigns only healthcare decisions to your named agent. While it is not the same as your financial power of attorney, you could choose the same capable, trusted individual to serve in both instances.
Although different advance directive forms may contain slightly different questions and formats, in general the document explains what treatments you want and don't want to achieve the quality of life that is important to you. This becomes an important guide for your healthcare proxy and medical team, who are obligated to follow it when you are near the end of your life and unable to speak for yourself.
You can change any part of your advance directive at any time while you are still able to communicate. In fact, you should review it every year or so and update it whenever your health or other circumstances change.