The Mafia is a group of families or clans in the criminal world that are loosely associated with each other. Also known as the Cosa Nostra, the Mafia originated in Italy in the mid-1800s and made its way to the United States later that century. Each family or clan controls the criminal activity of a certain territory of a city or locality. The Mafia has a set hierarchy that is respected across clans, and permission must be granted for one clan to do any type of activity in another family or clan's territory.
The Sicilian Mafia is a different entity than the American Mafia, though the roots of both organizations appear to be the same. The American Cosa Nostra — which translates into "This Thing Of Ours" — began in New York City and has spread across the country since. Each family is comprised solely of Italian-Americans, regardless of how far removed the bloodlines are. Both the Sicilian and the American Mafia operate under a code of conduct that prohibits members from fighting with each other, from having affairs with the wives of other members, and from other various insults within the family.
Turf wars have happened in the past to determine where one family's territory begins and another family's territory ends. Such turf wars were often bloody and symbolic, and after much fighting, New York City was divided into five families. Members of all crime families take an oath of silence, meaning they cannot talk to police about any family business, and if they do, they will be killed. Family and friends of the person who talked to police can also be killed as part of the Cosa Nostra code of conduct.
The American Mafia came to prominence during prohibition in the 1920s, trafficking alcohol all over New York City and even the country. The Sicilian Mafia was active and prominent long before that time, usually in a protection services capacity. They would essentially protect the interests of anyone who fell under their control or influence, whether that meant protection from competitive businesses, landowning pursuits, or other money-making enterprises. In return, the family would receive services and payments from those protected. Later on in its history, the Cosa Nostra would get involved in other moneymaking and power-grabbing operations, including drug and arms trafficking, election rigging, and buying of public contracts. These activities are less common in the American Mafia, but they are not unheard of.