The Federal RICO statute, which stands for Racketeer Influenced and Corrupt Organizations Act, was primarily designed to target organized crime (mafia) activities, but especially with the drug war, its application has become quite widespread.
In order to convict someone of RICO violations, the prosecution must prove beyond a reasonable doubt that at least two violations of 35 RICO qualifying crimes were committed within no more than a ten year period of time. There are 27 Federal laws and 8 State laws that qualify as RICO violations, for example, murder, extortion, bribery, narcotics trafficking, etc.
It should be noted that while the RICO statute has been used very successfully to break up criminal enterprises, it is most often used as to compel criminals to plead guilty to lesser offenses, or to testify against "bigger fish" in the criminal enterprise. This happened very recently when the FBI rounded up 120 members of organized crime based on the testimony of an informant they had indicted under the RICO laws.