"Mobster" is a slang term for someone who is part of the mafia or a mafia-like group. A mobster is a criminal. The word "Gangster" often means the same thing.
Images for kids
Sketch of the 1901 maxi trial of suspected mafiosi in Palermo. From the newspaper L'Ora, May 1901
Mug shot of Al Capone. Although never convicted of racketeering, Capone was a protégé and successor of Torrio, later convicted of income tax evasion by the federal government.