Omerta ("Silence") is a criminal organisation, apparently roughly based on the Mafia, which has grown up since the coming of the Hounds. However, they are a 'purified' or 'idealised' version of the Mafia.
They are led by the "Capo de Tutti Capi" (Leader of Leaders) whose identity remains a mystery. Local 'families' (lead by individual Capo) cover geographical areas, usually determined by communication times and population density. Thus there are allegedly many 'families' in the Halo, one for each planet (except Jupiter), one for each of Jupiters Trojan points, one for the Praxis Archipelago, one for the Tailchaser Islands, and eleven in the Asteroid Belt.
Omerta indulges in most criminal activities, the precise emphasis varying from family to family. These include, in some areas, wrecking - using false signal to draw ships into dangerous areas from where their cargoes and the can later be recovered.
All members are 'brainfixed' for loyalty, to kill them before they can talk (or have their minds read). However, before this they must prove their loyalty by committing crimes and undergoing a variety (of usually family-specific) entrance rituals and ordeals. Once an initiated Omerta member has proved themselves on at least one mission they acquire a nickname by which they are from that point known within Omerta. These usually bear a great deal of resemblance to the superhero names of the past.
Although the UN hunts Omerta, Omerta tends to avoid killing UN people if they can - the UN is enough trouble to them without becoming motivated by revenge too.
People join Omerta for many reasons. Some because they have no-where else to go. Others because they believe that Humanity is doomed, and no-one can save it, so they will do what they want, and at least live out their days in luxury or doing interesting things. A few are kidnapped or blackmailed into Omerta, people whose powers or abilities make them indispensable to Omerta. Some because they hate the UN and the Hounds, for example unregenerate members of dystopian societies which were forced to join the UN simply to survive.