Organized crime in real life is basically crime done for profit with a large group of criminals called a gang. It by default is done on the black market since it's illegal. Gangs can provide drugs, weapons, sex trafficking, and many other commodities.
It is considered one of the most dangerous forms of crime because of its scale, scope, and high level of violence the criminals involved are willing to inflict on those that displease them often involving death. Gangs have areas of operation called territories which is where they typically work. Gangs view certain people as trespassers should they go into their territory which are usually rival gang members.
Organized criminals tend to be armed with guns which makes them difficult to be taken down by police or anyone that has to defend themselves against them. In addition to each other gangs tend to have extensive connections with economic resources. They thrive in places with prohibition against precisely what they sell.
In Video Games
Organized crime in games gives the player many opportunities for things to do. Whether it be working for a gang or fighting one the player has many activities to participate in doing so. If the player is on good terms with a gang they will give them missions and rewards for completing them. If the player is on bad terms with a gang they will attempt to kill them on sight. This makes it difficult to transverse areas which are gang territory of a hostile gang.