A Will is the document that takes effect when you die.  Among many other things, it tells the world who will be looking after your estate and your young children, and how your estate is to be distributed.

A Power of Attorney is the document that is only valid while you are alive but are, in some way, incapacitated and cannot look after yourself or your property.  Among many other things, it tells the world who will be looking after you and your property and how you want to be cared for while you are alive.