POA's can also be drafted and executed in preparation for the future incapacitation of the party granting the POA. This type of POA is called a Durable Power of Attorney. Many people will execute a Durable POA prior to an upcoming major surgery or when they are concerned about the onset of Alzheimer's or other incapacitating disease. Durable POA's are often executed in conjunction with a Living Will.
A Power of Attorney can be either General or Limited. A General Power of Attorney grants permission for the agent to act on behalf of the principal in all matters in general. A Limited Power of Attorney grants permission for the agent to act on behalf of the principal in a defined matter or set of matters. Read my post on Agency for more information on the rights and responsibilites of an agent.