Taxation in the US comes in many different varieties. We have taxation on income, self-employment, gifts, and so on. When speaking in terms of the estate tax, we are discussing a tax imposed on a transfer of net assets, a transfer tax.
Different government bodies impose various types of taxes. Federal tax governs all US citizens or domiciled residents and encompasses all the different states found across the nation.
Each US state also has its own set of rules and regulations that apply to specific residents of that particular state.