What is a qui tam whistleblower lawsuit?

The False Claims Act contains a provision allowing whistleblowers, or qui tam “relators”, to bring suits on behalf of the United States against the false claimant.  “Qui tam is short for the Latin phrase qui tam pro domino rege quam pro se ipso in hac parte sequitur, which means ‘who pursues this action on our Lord the King’s behalf as well as his own.’” Vt. Agency of Nat. Res. v. United States ex rel. Stevens, 529 U.S. 765, 768 n.1 (2000).