A racketeer is a criminal, especially one who's involved in organized white collar crime or dishonest business dealings. If a lawyer embezzles money from her clients, she is a racketeer.
Racketeers commit crimes in order to make money for themselves. In many cases, a racketeer is a business owner or professional who falsifies financial records to hide income or bills clients for more than they owe. A racket is a scam or fake service that's run by a racketeer — and the crime itself, if the racketeer is caught, is called racketeering. The source of all three words is thought to be racquet, and the idea of a scam as a "game."