What Exactly Defines a Slum?
Every informal settlement has a unique set of conditions- no two are alike. The United Nations Human Settlements Programme (UN Habitat) defines slums mainly via physical conditions- social and economic factors play no part. 
In fact, many informal settlements have strong, organized social systems and are hotbeds of entrepreneurial activity. Many of these communities are embedded with multiple income tiers, middle class professionals, yet the conditions in which people must live are insufficient.
For further info on characteristics of slums, see David A. Smith's Affordable Housing Innovations, Issue 06 here.