Business Insider has this amazing slide of tour of World Largest Slums. The slums toured were Altos de Cazuca in Colombia, Comuna 13 also in Colombia, Kamagasaki in Japan, Ashaiman slum in Japan, Rocinha in Brazil, Sultanbeyli in Turkey, Petare in venezuela, Dharavi in India, Sadr City in Iraq, Orangi town in Pakistan, Neza-Chalco-Itza barrio in Mexico and Kibera in Kenya.
Most of the slums are inhabited by poor fighting for economic space in big cities. Some of them are home to industries and some to gangs. Some very much the centre of terrorist cells. It was amazing to see the satellite picture below of Kibera.
The UN Habitat report states that 830 Million of the 3.5 Million city dwellers live in slums. You can get to tour the slums here. Kibera is said to be home to more than 200,000 inhabitants.