Healthcare Directives go by various names such as advanced directives or a living will. If you are confused about the difference, do not worry because many are confused. There is no difference except for each particular state calls these directives.
A basic healthcare directive is allowing another person, usually a relative, to make healthcare decisions for you. These are used when the grantor, the one who gives the power to make the decisions, can no longer make the decisions necessary due to incapacity.
These go beyond just granting the power to make decisions but may also specify specific instructions to follow. Famous cases involve "Permanent Vegetative States" where the grantor is without brain function but may still have body function. This where the Grantor may specify that they do not want to live on machines or how long they want to be in that state before making a decision.
A healthcare directive can do much good for the family of the grantor by specifying specific wishes or allowing one person to make the decisions necessary. This prevents a family from guessing and fighting about what they think that the Grantor would want in any serious scenario.