A Will is a legal document that allows you to make enforceable decisions about how your estate will be distributed following your death. It is the only way to formally ensure that your property is distributed according to your wishes.
A Power of Attorney is a document allowing you to appoint another person (your attorney) to act on your behalf in relation to financial, property, personal and health matters in the event that you are unable to do so. The document also allows you to set conditions in relation to refusing or requiring the withdrawal of medical treatment.
Estate planning is more than just making a plan to distribute certain assets to certain people. A complete estate plan will allow you to retain control of your assets and to determine who will make decisions on your behalf should you become unable to in the future.