Gangsters are members of a professional crime organization, i.e. a gang.
Famous examples of such organizations include the Mafia, Crips and Yakuza.
Gangsters typically run their operations as a pseudo-business in that they do offer some product or service, albeit an illegal one, for paying customers, rather than outright theft, although they engage in plenty of that as well. For example during the prohibition era, gangsters monopolized the alcohol trade, in the 1950s it was gambling, and today it is narcotics.
Other classic gangster endeavours include prostitution and charging local businesses "protection money" as if the gang were a private security firm, when in fact the payments are made solely to protect the business from the gangsters themselves and sometimes from other gangs. In other words indirect armed robbery or extortion.
They frequently take over or wield undue influence in labor unions.
Gangsters also are known for attempting to manipulate the outcome of civil institutions, such as court cases and political elections, through bribery and intimidation. When gangsters become particularly powerful, they may eventually develop reciprocal relationships with law enforcement they have managed to corrupt. In this situation the police are handsomely paid off, and in exchange they ignore their patron's illicit activites, and may even assist them, by directly taking part in crimes, or by arresting competitors.
Today an Ebonics form of the word, "gangsta", has become associated with gangsta rapmusic. This style's lyrics are often based on living gang-related lifestyles, and can sometimes be portrayed in a realistic, gritty way, or in a cartoonish, exaggerated way.