A power of attorney is a legal document that gives someone you choose the power to act in your place.
In case you ever become mentally incapacitated, you'll need what are known as "durable" powers of attorney for medical care and finances. A durable power of attorney simply means that the document stays in effect if you become incapacitated and unable to handle matters on your own. (Ordinary, or "nondurable," powers of attorney automatically end if the person who makes them loses mental capacity.)
With a valid power of attorney, the trusted person you name will be legally permitted to take care of important matters for you -- for example, paying your bills, managing your investments, or directing your medical care -- if you are unable to do so yourself.
Taking the time to make these documents is well worth the small effort it will take. If you haven't made durable powers of attorney and something happens to you, your loved ones may have to go to court to get the authority to handle your affairs.
To cover all of the issues that matter to you, you'll probably need two separate documents: one that addresses health care issues and another to take care of your finances. Fortunately, powers of attorney usually aren't difficult to prepare.