Corrido stands for ballad, and narco, of course, involve drugs.
Kind of gangstarap alla mexicana, wich means that in fact it's 'gangstamariachi'.
The songs, based on polkas and waltzes, feature lyrics backed by accordions and brass bands.
Narco corridos, had spread to California, Texas, Nevada, Florida and other places with large Hispanic populations (Colombia, El Salvador, etc).