The Internal Revenue Service (IRS) is the United States federal government agency that collects taxes and enforces the internal revenue laws. It is an agency within the U.S. Department of the Treasury and is responsible for interpretation and application of Federal tax law. The official U.S. Treasury regulations provide (in part):
The Internal Revenue Service is a bureau of the Department of the Treasury under the immediate direction of the Commissioner of Internal Revenue. The Commissioner has general superintendence of the assessment and collection of all taxes imposed by any law providing internal revenue. The Internal Revenue Service is the agency by which these functions are performed.
– 26 C.F.R. section 601.101(a).
Source : en.wikipedia.org/wiki/Internal_Revenue_Service