The forfeiture statutes -- 18 U.S.C. § 1956(c)(7), 28 U.S.C. § 2461(c), and 18 U.S.C. § 981 -- create a complex, interlocking web. But in simple terms, any assets derived from proceeds traceable to a violation of federal law, or a conspiracy to violate federal law, can be forfeited.
Criminal forfeiture is intended to punish convicted defendants. It also rewards law enforcement agencies. The U.S. Marshals Service says, "The proceeds from the sale of forfeited assets such as real property, vehicles, businesses, financial instruments, vessels, aircraft and jewelry are deposited into the AFF [DOJ Assets Forfeiture Fund] and are subsequently used to further law enforcement initiatives." The Marshals now have around $2 billion in forfeited assets under management.
Once there's a criminal conviction in a case with a forfeiture count, there's not much the defendant can do. The Legal Information Institute says:
The nature of the proceeding assures that the defendant is protected by the procedural rights embodied in the Fourth and Fifth Amendment. The property must be identified in the indictment in order to serve notice to the defendant, and opportunity must be given to contest the forfeiture. Although the [criminal] conviction requires the government to prove guilt "beyond a reasonable doubt," the forfeiture is subject to a lower burden--preponderance of the evidence. Furthermore, the burden shifts to the defendant once the government shows that the defendant acquired the property around the time of the crime, and no other likely source existed.
Defense of forfeiture is not impossible, but the laws have been so structured as to make any defenses extremely complicated and expensive.