In a qui tam action, a private party called a relator helps the government prosecute a defendant. In return, if the government wins, the relator receives a share of the award. In qui tam actions, the government, not the relator, is considered the real plaintiff.
The federal False Claims Act authorizes qui tam actions against parties who defrauded the federal government. 31 U.S.C. § 3279 et seq. If successful, relators in False Claims Act qui tam actions may receive up to 30% of the government's award. See False Claims Act.
See, e.g., United States ex rel Eisenstein v. City of New York (08-660).