The Internal Revenue Service is the United States government branch that collects taxes and enforces the tax laws. It is a part of the United States Department of the Treasury.
For most of its first 138 years (1776-1913), the United States did not have a federal income tax. An income tax was enacted during the latter half of the 19th century, but it was declared unconstitutional by the United States Supreme Court. In 1913 the United States Constitution was amended (see Sixteenth Amendment) to permit the enactment of an income tax, which was promptly accomplished and remains in effect in an increasingly complicated form. Prior to the establishment of the income tax, all government taxes were collected as excise taxes on products, import/export taxeses, and revenue taxes on items such as photographs, bank cheques, stock certificates, wills, deeds, and other legal documents.
Also see Taxation in the United States.