Chief Justice Hughes wrote for a unanimous Court in invalidating the industrial "codes of fair competition" which the NIRA enabled the President to issue. The Court held that the codes violated the constitutional separation of powers as an imperssible delegation of legislative power to the executive branch. The Court also held that the NIRA provisions were in excess of congressional power under the Commerce Clause.
The Court distinguished between direct effects on interstate commerce, which Congress could lawfully regulate, and indirect, which were matters of purely state law. Though the raising and sale of poultry was an interstate industry, the Court found that the "stream of interstate commerce" had stopped in this case--Schechter's slaughterhouses bought chickens only from intrastate wholesalers and sold to intrastate buyers. Any interstate affect of Schechter was indirect, and therefore beyond federal reach.
Though many considered the NIRA a "dead statute" at this point in the New Deal scheme, the Court used its invalidation as an opportunity to impose limits on congressional power, for fear that it could otherwise reach virtually anything that could said to "affect" interstate commerce and intrude on many areas of legitimate State power.
Justice Cardozo's concurring opinion clarified that a spectrum approach to direct and indirect effects is preferable to a strict dichotomy. Cardozo felt that in this case, Schechter was simply too small a player to be relevant to interstate commerce.
This narrow reading of the Commerce Clause was later disavowed by the Court, which began to read congressional power more expansively in this area. However, more recent cases such as United States v. Lopez perhaps signal a growing inclination in the Court to once again impose limits on its scope.