Article Three of the United States Constitution
Clause 2 says that the Supreme Court has "judicial power [over] all Cases, in Law and Equity, arising under this Constitution [and] the Laws of the United States...."  However, it does not say exactly what this means.  The Supreme Court clarified this in a case called Osborn v. Bank of the United States (1824).  Supreme Court Justice John Marshall wrote an opinion about what a "federal" case is, and what "arising under" the Constitution or federal laws means.  Basically, he said a case is federal  and under the federal courts' original jurisdiction  if: