A health care power of attorney allows the agent to make decisions about the medical treatment for the principal, but only if the principal is unable to make such decisions for him or herself.
A living will is a health care power of attorney that is limited to the the refusal of certain medical treatment in the event of terminal illness or injury. Without a living will, doctors may decide they are legally obligated to perform certain procedures which you may not desire, in the event you become seriously ill and are unable to communicate your desires. Sometimes a living will is referred to as an "advance directive" or "do not resuscitate (DNR)".
These law review articles give opinions as to what the authors think the law should be, not necessarily what the law currently is.