its not organized like the mafia, this is more like a lable for thousand of cliquas in Central America and US. The Mara Salvatrucha (i would say that means, salvadorian gang), originated in LA. It was founded my Salvadorians to defend themselves against others (mexicans). Nowadays its international. 13 comes from the city code of LA, and is the gang code for the surtenos, generelly gangsters from southern calif/LA and the cities (in prison they are organised in la EME=the Mexican Mafia).