A criminal organization is an organization whose actions violate law. Some criminal organizations, such as terrorist organizations, act under political motivations. Mafias are criminal organizations whose primary goal is to make a profit.
Criminal organizations obviously keep most of their actions as secret as possible. Communication between members is primarily done through word of mouth. The best way to obtain information about them is often to infiltrate them.
In order for a criminal organization to prosper, some degree of support is required from the society in which it lives. Thus it is often necessary to corrupt some of its respected members, which is most commonly achieved through bribery, blackmail, and the establishment of symbiotic relationships with legitimate businesses. Also, financing is made easier by the development of a customer base inside or outside the local population, as occurs for instance in the case of drug traffic.
Lacking much of the paperwork that is common to legitimate organizations, criminal organizations can usually evolve and reorganize much more quickly when the need arises. They are quick to capitalize on newly opened markets, and quick to rebuild themselves under another guise when caught by authorities.
Globalization occurs in crime as much as it does in business. Criminal organizations easily cross boundaries between countries. See for instance this article on transnational criminal organizations.
explain how and why criminal organizations appear, why people join them, how government and other organizations attempt to fight them