When someone passes, his or her property must be distributed through probate. The process is generally overseen by an executor, if there is a will, or by a court (and a court appointed personal representative) if there is no will. An executor is the person designated to administer the estate - this person is also known as the personal representative. Most jurisdictions require that the executor post a bond to protect the assets of the estate.
Probate involves identifying and inventorying the deceased person's property, accounting and appraising the property, and then paying taxes and creditors with the deceased's assets.
If there is a will, the assets are distributed according to the instructions of the will. If not, then state law determines who gets what and how much.
Having a will, alone, does not mean probate is unnecessary. Although a will might make the process simpler, probate is still required for assets in the deceased's name alone.
In general, property which the deceased owned individually has to pass through probate for ownership to pass to his or her heirs. Jointly owned property and the proceeds of life insurance, retirement accounts, and annuities pass to the surviving joint owner or the named beneficiaries without the necessity of probate.