As the term is commonly used in California, an Advance Directive is primarily two separate documents in one—a "Directive to Physicians" (sometimes called a "Living Will") and a "Durable Power of Attorney for Health Care." It may also contain other provisions. Let us look at each of those parts separately:
Directive to Physicians — This is a document that lets you specify how you want to be treated in various life-threatening situations where you are so incapacitated that you can no longer make your wishes known. Do you want to be kept alive, no matter what-for example, even if it is determined that you are "brain dead" and in a permanent "vegetative state"? Or, would you rather be disconnected from life support if all it is doing is simply prolonging the moment of your death? What about drugs and pain? Would you rather be kept pain free, even if it can lead to drug dependence or hasten your death? Or do you want to be kept alert and drug free as long as possible? What if your heart stops beating? Do you want to be resuscitated or simply let nature take its course? What if you are physically healthy but lose your mental capacities due to Alzheimer's Disease or other forms of dementia, or even to sudden head trauma? What kind of facility do you want to be placed in, if you could no longer live at home? What type of care do you wish to receive? These are types of decisions that physicians and family members often have to make in the absence of explicit instructions from their loved one, and the psychological and emotional strain that places upon them is usually something most individuals would not wish on their worst enemy. Without an Advance Directive, doctors are forced by law to always err on the side of caution, to keep you alive even if you would rather not be maintained artificially.
Durable Power of Attorney for Health Care — Not all possible contingencies can be thought out in advance. Sometimes life-and-death decisions must be made immediately. If you're not conscious or otherwise able to make those decisions about yourself, who should? Most people have one or two trusted persons whom they would like to take over in such a situation. With a Durable Power of Attorney for Health Care, you can specify just whom you want to act as your personal agent (sometimes called an "attorney-in-fact") to meet and confer with your doctors and make all those decisions that you have not already specified in advance.
If you have no legal documentation in place, then certain state laws take control, and the persons having control over your destiny may not be the ones you would have chosen. Two cases in point: would you want an estranged, but still legally married, spouse to be the one to make medical decisions for you? Or what about a parent or sibling with whom you've hardly communicated in decades, over the objections of your life partner (but not a legally-married spouse) for the past 25 years?
Although a Durable Power of Attorney for Health Care and a Directive to Physicians are often included in the same document (and the combination called an Advance Directive), they do not have to be combined, and may, in fact, exist as two separate documents.
By the way, the word "durable" in "Durable Power of Attorney" is there because a durable power of attorney is a document that endures your incapacity, that is, it stays in effect indefinitely—either until you die, or until you recover sufficiently to regain control over your own affairs.