Internal Revenue Service
The IRS is the part of US government that is responsible for collecting taxes. Tax law is set by Congress. The IRS merely enforces that law and facilitate the collection of the taxes.
In 1913 states voted to amend the constitution to allow a federal income tax. This became the 16th Amendment and is the basis for the existence of the Internal Revenue Service. Originally the tax rate was 1% on incomes above $3000 and 6% on incomes over $500,000.