A will is a testamentary document that speaks at your death as to how, where and to whom you want your estate to pass. You can include the name of a person you wish to take care of your minor child(ren) and/or their estate, which is called a Guardian.
A will is a static document, which means it only goes into effect after your death. It can not direct or speak to anything during your life. A will must be probated in the court. It is subject to notice requirements and the judge's approval before any final distribution can be made. To be valid, wills must be created and signed with certain formalities. See Amendments to understand how your will can be changed.