Across the globe, 1.8 billion people -- a quarter of the world's population -- work off the books each day. They are paid in cash for the goods they sell and the services they provide, and due to their ubiquity, there's a word for these merchants in nearly every language. As Robert Neuwirth reports, in French colonies, they're known as débrouillards -- self-starters, entrepreneurs, all outside the bureaucratic system. They might be vendors selling revolutionary goods in Egypt's Tahrir Square, Nigerians selling mobile phones, or the guy down the street hawking flowers on the corner. Whoever they are, they work in the world's fastest-growing economy: System D.