As per the Constitution of The United Sates section 8 paragraph 1:
The Congress shall have Power To lay and collect Taxes, Duties, Imposts and Excises, to pay the Debts and provide for the common Defence and general Welfare of the United States; but all Duties, Imposts and Excises shall be uniform throughout the United States;
So, Congress has passed into law the United States Code.
United States Code TITLE 26--INTERNAL REVENUE CODE
SUBTITLE A--INCOME TAXES
CHAPTER 1--NORMAL TAXES AND SURTAXES
SUBCHAPTER A--DETERMINATION OF TAX LIABILITY
PART I--TAX ON INDIVIDUALS
Sec. 1. Tax imposed.
http://frwebgate.access.gpo.gov/cgi-...d=Cite:+26USC1
I hope that helps.