What are some differences and similarities between formal and informal urban settlements? Why do each develop, and what are some of the different forms that they take?

Robinson et al. (2016) chapter and watching the Neuwirth (2005) video:

What are some differences and similarities between formal and informal urban settlements? Why do each develop, and what are some of the different forms that they take?

Neuwirth, R. (2005, July). The hidden world of shadow cities [Video]. TED Conferences. https://www.ted.com/talks/robert_neuwirth_on_our_shadow_cities?language=en
Robinson, J., Scott, A. J., & Taylor, P. J. (2016). Chapter 2: Cities in time and space. In Working, housing: Urbanizing. Springer. http://link.springer.com/book/10.1007%2F978-3-319-45180-0