An estate tax is in force when there is a transfer of an estate (property) of a deceased person. The estate may be transferred by will, under the law of intestacy, from an intestate trust, or even the payout of certain life insurance benefits, etc. The estate tax is a federal law.
The other part of the overall system, the gift tax, levies taxes on property transfers while a person is still alive. In other words, the gift tax prevents people trying to avoid the estate tax if they want to give their estate away. In order to ensure either of these situations is dealt with properly, estate tax planning is in order. For instance, if an estate asset is left to a federally recognized charitable organization, the tax may not apply. Estate taxes may not be charged if a married couple passes up to $10,000,000 or an individual passes $5,000,000.
Over and above the federal government law, many states also have an estate tax, usually referred to as an inheritance tax, or the death tax, by those that oppose being taxed twice on their death.