Simply put, a Will is any legal document in which you declare who will manage your estate, receive specific belongings, and become the guardian of any dependents you may have. However, the particulars of your Will may vary greatly depending on what outcomes you hope for, and how large and complex your estate is. For a Will to be validly executed, it must be submitted to the courts.
Without a Will, state laws will dictate who inherits what, but this can often lead to complications and outcomes you likely would not want for your friends and family. Your will could also end up being contested, in which case you will want it to be clearly drafted and validly executed so it will hold up in the courts. For all these reasons and more, it is strongly recommended you work with an experienced lawyer like myself.