Organized crime in Italy has traditionally involved three major groups from southern Italian regions: the Mafia in Sicily, the Camorra in Naples and the 'ndrangheta in Calabria, all of which came to prominence following the northern-led unification of Italy in the mid-19th century, in an area already afflicted with considerable bandit activity, although the roots undoubtedly go rather farther back (the suggested etymologies given in other writeups are rather fanciful, though). In recent years a parallel grouping has appeared in Puglia, the Sacra Corona Unita ("united sacred heart"). Although they are all involved in the usual range of criminal operations - anything that will turn a profit - the 'ndrangheta were particularly active in the wave of kidnapping in the 1970s and 1980s, while the Camorra were involved with corruption at central government level, particularly through the former Christian Democrat (DC) party, and had substantial involvement in the cement and construction industry, which together led to the building of rather a lot of pointless infrastructure throughout the Mezzogiorno and by siphoning off emergency relief funds has severely compromised the reconstruction work following recent earthquakes.
The Sicilian Mafia was successfully suppressed during the Fascist ventennio, but was actively revived with the connivance of the United States government (acting through links with the American mafia with Lucky Luciano in a key role) during World War II in return for assistance during the Allied invasion of Sicily (1943) - as a result the American 5th Army landed virtually unopposed while the British 8th Army faced a stiff fight. It took a firm hold in the corruption-rife Italian post-war governmental system and, with a few hiccups, has never looked back since, managing effectively to subvert the best intentioned judicial manoeuvres through bribery, murder and FUD; arrested mafiosi tend to "repent" in ways that take the courts years to disentangle.
For literary coverage of the Italian mafia, the reader is recommended to take a look at the works of Leonardo Sciascia (eg Il Contesto - The trial, and Una storia semplice - A simple story), or less literarily the relevant Aurelio Zen novels by Michael Dibdin and, of course, Mario Puzo.