Although there are overlaps, the Mob and the Mafia are not the same. In the broadest sense of the terms, the Mafia can safely be described as a mob, but the Mob is not always associated with the true Mafia. The original crime organization known as the Mafia or Cosa Nostra began in Sicily, Italy, and membership is strictly limited to native Sicilians. The Mob has no such restrictions, which is why notable non-Sicilian gangsters such as Al Capone and Meyer Lansky could be considered mobsters, but not members of the official Italian Mafia.
Besides Sicilian heritage, another difference is organization. The original Mafia created a very specific chain of succession, which some organized crime experts suggest was inspired by the Roman Catholic Church's governing system. Rank-and-file members would have to work their way through the ranks in order to earn more respect from their superiors and gain more power within the organization. A mob, on the other hand, may not have a very well-established hierarchy. Leadership could change in an instant through sheer force or assassination.
This is not to say the differences between the Mob and the Mafia are always readily apparent to outsiders. Both organizations derive their income through illicit proceeds from loan sharking, prostitution, gambling, protection money and other unsavory dealings, but the Mob in general is more nebulous and vague than the true Mafia. Mafia bosses and their subordinates often become known entities to local and federal law enforcement agencies. They largely avoid prosecution by delegating illegal activities to unknown associates and by maintaining scrupulously legal lifestyles in public.
Leaders of an organized mob, however, often remain underground and secretive. Their orders are frequently passed through trustworthy intermediaries and couriers. While a Mafia capo de capo, or top man in the organization, may enjoy his public notoriety, the leader of a mob is far less likely to become so visible or notorious.
Another difference could be national or cultural origin. The original Sicilian Mafia maintained a strict policy against admitting non-Sicilians to their ranks, but that did not stop other Italians from organizing an Italian mob. This same philosophy held true for native Russian, Japanese and Irish criminals, who all formed strong organized crime syndicates in their respective countries. Some may use terms such as the Irish Mafia or the Russian Mafia to describe these groups, but others argue that "Mafia" should only be applied to the original Italian organization.
The difference between the Mob and the Mafia is not just a matter of semantics, but the two terms can often be used interchangeably in conversation. In organized crime circles, however, there may be a palpable difference between joining an ethnic mob and joining the Mafia. Association with a mob may be an informal arrangement, but membership in the official Mafia is typically a lifetime commitment. Defection is strongly discouraged, although loyalties between mobs have been known to shift.