An Advance Healthcare Directive is a legal document in which a person specifies what actions should be taken for their health if they are no longer able to make decisions for themselves because of illness or incapacity. In the U.S. it has a legal status in itself, whereas in some countries it is legally persuasive without being a legal document.
A Living Will is one form of Advance Directive, leaving instructions for treatment.
Another form is a specific type of Power of Attorney or Healthcare Proxy, in which the person authorizes someone (an Agent) to make decisions on their behalf when they are incapacitated.
People are often encouraged to complete both documents to provide comprehensive guidance regarding their care, although they may be combined into a single form.