Antitrust law is the study of competition. It is the body of law that seeks to assure that players in the marketplace do not take actions that interfere with the functioning of competitive markets.
Antitrust is primarily federal law and arises under two federal statutes: Sections 1 and 2 of the Sherman Antitrust Act, 15 U.S.C.§§1-2, and §7 of the Clayton Act, 15 U.S.C.§18. The Sherman Antitrust Act and the Clayton Act are supplemented by three additional acts. The Federal Trade Commission Act of 1914 (as amended in 1938 and 1975), 15 U.S.C.§41 et. seq., which created the federal Federal Trade Commission agency, and the Robinson-Patman Act of 1936 (or Anti-Price Discrimination Act) 15 U.S.C.§13 et. seq., which limits the ability of sellers to discriminate against different buyers (i.e., charge different prices for the same goods), and the Hart-Scott-Rodino Antitrust Improvements Act of 1976, 15 U.S.C.§18a.
The Department of Justice's Antitrust Division Manual summarizes the main antitrust laws.