Joint Committee on Taxation, Overview of the Federal Tax System as in Effect for 2012 (JCX-18-12):

This document … provides a summary of the present-law Federal tax system as in effect for 2012. The current Federal tax system has four main elements: (1) an income tax on individuals and corporations (which consists of both a “regular” income tax and an alternative minimum tax); (2) payroll taxes on wages (and corresponding taxes on self-employment income) to finance certain social insurance programs; (3) estate, gift, and generation-skipping taxes, and (4) excise taxes on selected goods and services. This document provides a broad overview of each of these elements.