A power of attorney is an instrument by which the "principal" (the person signing the POA) appoints an "agent" or "attorney-in-fact" and grants the agent certain specified powers. A POA can be general (meant for all purposes) or limited (meant for specific purposes such as selling a home or accessing a particular bank account).
The term "Durable" when used with a POA means the POA continues until it is revoked or the principal dies. A POA that is not durable would become ineffective if the principal is ever incapacitated. In practice, virtually all POA's are durable.
There are certain tasks for which the POA must specifically authorize, such as making gifts (gifting without permission is stealing), signing and funding trusts, etc. The agent under a POA is a fiduciary, sort of like the person standing behind the counter at the bank. The agent isn't in charge and can be made to account the principal (or the principal's heirs) for whatever actions the agent has undertaken.
A POA can have immediate effect or can be drafted to come into effect upon the incapacity of the principal.
As a rule, never name someone an agent in a POA unless you trust that person 110%.
