A power of attorney is a legal document wherein you give another person the right to act on your behalf. It is a very powerful document, in that your agent can act as you. For example, once in effect, the agent may be able to access your bank accounts, pay bills, transfer real estate, make gifts on your behalf, and conduct your business and affairs. While the agent has a fiduciary duty to make sure that he is acting on your behalf, you need to choose an agent (or two) who you strongly trust.
A power of attorney may be in effect immediately, or it may be a springing power of attorney, meaning that it is only in effect when you are incompetent and unable to make decisions on your own. Most powers of attorney are durable instruments, in that they do not terminate upon incapacity.
If you wish to revoke a power of attorney, you must notify your agent in writing. Otherwise, the power of attorney remains in effect until you pass away.