Your Will, or, more formally, your Last Will and Testament, is the legal document in which you direct how and to whom your property, both real and personal, will be distributed after your death.
In your Will, you name a personal representative (often referred to as an Executor) who will be responsible for handling the probate administration of your estate in accordance with your stated wishes.
If you have minor children, your Will is where you appoint legal guardians and/or trustees for any of your children who are minors at the time of your death. The legal guardian is the person you choose to be responsible for the raising of your children and the trustee will be responsible for the management and distribution of the estate assets you have left to your minor children.
If you have a trust, you still should have a pour-over Will, which puts your probate assets into the trust to be distributed according to the terms of the trust.