Wills are the most well know estate planning document.  A Will allows you to direct the manner in which your property - be it real estate, cash or personal property - will be distributed after your death. Wills are subject to the jurisdiction of the Probate Court and the process dictated thereby. If you die without having a Will, that process is still determined by the Probate Court, however, the distribution of your property is determined by the state's laws of intestacy, which may or may not reflect your personal wishes.

A Will can also provide an opportunity to for you to nominate a Guardian to care for your minor children and a Custodian to manage assets for said minor children.  You can also chose an Executor who will manage your estate through the probate process.  The Executor will be responsible for gathering your assets, paying your debts and distributing any remaining assets to those you have chosen to receive it. 

A Will can effectively set forth your wishes up on your death.  However, a Will goes into effect once you die, so it does not provide a plan if you were to become incapacitated and need someone to manage your affairs during such incapacity.  In such cases, a Trust may be an appropriate part of your estate plan.