Can be anybody who commits crimes from a street hood to a mobster.
Anybody who commits crimes is called a criminal. There's a difference. Gangster was originally a term to describe Italian-American mobsters in the 1920s till now. As you may know, Italian American mobsters were involved in organized crime. There's a difference between a person involved in organized crime and a person involved in street crime, or gang activities. The fact that the Crips or Bloods are called 'street gangs' does not mean that the word 'gangster' applies to them, but instead the word 'gang member' is more appropriate, even so being a formal term. The more informal term people have come up with is 'gangsta'. I find this to be a cheap rip off of the word 'gangster' and when a person says 'I'm gangsta, yo.' I do not take them serious even if they have a gun or belong to a gang. Street gangs are commonly associated with the hip hop subculture, such as in most street gangs, major and minor ones, we see people with hip hop clothing, ranging from oversized t-shirts to du-rags and bandanas as well as 'bling bling' and oversized baggy pants that are left hanging down to expose a gang member's unwanted sight of his underwear or ass crack, even though they wear belts to go along with it. But not just the clothing, the slang is the same. Street slang and hip hop slang as well. Members of street gangs generally listen to the hip hop music genre. They generally live in the ghetto/housing projects and have begun to expand into suburban and rural settings. They usually eat out at fast food resturaunts such as McDonalds and the like.
As for gangsters from organized crime, depending on the group, they usually live in either middle class or upper class neighbourhoods and make five or six figure annual salaries, and drive expensive cars. This is also defined by the group. For example, during the 1960's, Italian American mobsters perferred to drive Cadillacs. Russian mobsters, currently, for example, as well as other Eastern European and Balkan organized crime figures, perfer to drive luxury vehicle brands such as Mercedes, BMW and Audi, as I see with Slave Angel Ivanovski and Ljube Mandzuko, the owners of Hotel Oaza in Stip, Macedonia and many other locales around the Balkans and elsewhere, they have Mercedes and BMW cars, including one which I love and admire, the Mercedes CLS350. I love this car. They usually dress in formal clothing such as business attire like suits and tuxedos, but the informal clothing usually involves anything from sweat suits to leather jackets and jean pants to dress shirts and regular casual clothing. The also usually wear expensive jewelry such as gold rings and watches, and perfer brands such as Rolex for watches, for example. They listen to all types of music, depending on where the person is from. They eat out everywhere, depending on the mobster's income, from fast food resturaunts to classy fine resturaunts.
Of course, these are just stereotypes. Anyone who commits crimes is a criminal. Anyone who commits sexual crimes is a sexual offender. Anyone who commits organized crime is considered a gangster, but this being that he commits more than one type of organized crime and belongs to a group involved with organized crime, this is where the term 'Mafia' comes in, as they base the organized criminals' group on their background, so if they are Russian, they call it the Russian mafia, Italian --- Italian mafia. Anyone who is a member of a gang is called a gang member. This does not make him a gangster. Gangster and mobster are both more formal and informal terms. Gangsta is not formal, more informal, and is thus slang.
Alas, once again, these are stereotypes per se. Look it up in the dictionary and the general term will tell you the correct meaning as there are more than one meanings to a word sometimes.