The Federal Government
Visualize a three branched tree. Congress, the legislative branch of the federal government, makes the tax law. The executive branch, which includes the Treasury Department, administers the tax law through the IRS. The judicial branch comprises all the federal courts, which interprets the tax laws and overrules the IRS when it goes beyond the law.
The power to tax incomes was granted by the 16th amendment to the U.S. Constitution; the first income tax act was passed in 1913. Contrary to what fringe groups and con artists contend income tax law and the IRS are legal and are not going away.
Tax law begins with the internal revenue code. Congress enacts and revises the tax code. The president signs it (usually), and it becomes law. One major reworking of the IRC was officially called the Tax Reform Act, but was known to tax pros as the Accountants and Tax Attorneys Relief Act. The tax code is now over 8,500 pages of exceedingly fine print.
The Internal Revenue Service is a division of the Treasury Department. It is headed up by the commissioner of Internal Revenue, a presidential appointee. the irs is changed with enforcing the tax code.
The United States Tax Court interprets the tax laws and decides diputes between the IRS and taxpayers and interprets the tax code. It is pretty easy to go to the tax court in most cases, even without an attorney. Tax disputes are also decided in U.S. District Courts and the Federal Court of Claims, but these require payment of the disputed tax first, unlike in the tax court.