The structure described below refers specifically to La Cosa Nostra. Other groups have similar structures, but they may differ in some ways.
Each group is made up of several gangs, known as families. The number of families can range from fewer than 10 to more than 100. Sometimes, the emergence of a new family must be approved by the heads of other families, while in some cases a group can splinter off from another family and consolidate its power, becoming recognized as a new family over time. Each family has separate business dealings, but the dealings of the families can intermingle to a large extent depending on their proximity to one another and the commonality of their ventures.
The leader of each family is known as the boss, or don. All major decisions are made by the boss, and money made by the family ultimately flows to him. The boss's authority is needed to resolve disputes and keep everyone in line.
Just below the boss is the underboss. The underboss is the second in command, although the amount of power he wields can vary. Some underbosses resolve disputes without involving the boss. Some are groomed to replace the boss if he is old or in danger of going to jail.
Beneath the underboss are several capos. The number of capos varies depending on the overall size of the family. A capo acts like a lieutenant, leading his own section of the family. He has specific activities that he operates. The capo's territory may be defined geographically (as in, "everything west of 14th Street belongs to Louie 'The Key' DiBartolo.") or by the rackets he operates ("Alfonze 'Big Al' Maggioli is in charge of illegal gambling."). The key to being a successful capo is making money. The capo keeps some of the money his rackets earn and then passes the rest up to the underboss and boss.
The "dirty work" is done by the soldiers. A soldier is the lowest rank among made men. They're part of the family, but they hold little power and make relatively little money. The number of soldiers that belong to any given capo can vary tremendously.
In addition to soldiers, the Mafia will use associates. Associates are not actual members of the Mafia, but they work with Mafia soldiers and capos on various criminal enterprises. An associate is simply someone who works with the mob, including anyone from a burglar or drug dealer to a lawyer, investment banker, police officer or politician.
There is one other position within the family that is somewhat legendary -- the consigliere. The consigliere is not supposed to be part of the family's hierarchy. He is supposed to act as an advisor and make impartial decisions based on fairness rather than personal feelings or vendettas. This position is meant to elected by the members of the family, rather than appointed by the boss. In reality, consiglieres are sometimes appointed and are not always impartial.