A slum is a highly populated urban residential area consisting of densely packed housing units of weak build quality and often associated with poverty. The infrastructure in slums is often deteriorated or incomplete, and they are primarily inhabited by impoverished people. Although slums are usually located in urban areas, in some countries they can be located in suburban areas where housing quality is low and living conditions are poor. While slums differ in size and other characteristics, most lack reliable sanitation services, supply of clean water, reliable electricity, law enforcement, and other basic services. Slum residences vary from shanty houses to professionally built dwellings which, because of poor-quality construction or lack of basic maintenance, have deteriorated.

A shanty town, squatter area or squatter settlement is a settlement of improvised buildings known as shanties or shacks, typically made of materials such as mud and wood. A typical shanty town is squatted and in the beginning lacks adequate infrastructure, including proper sanitation, safe water supply, electricity and street drainage. Over time, shanty towns can develop their infrastructure and even change into middle class neighbourhoods. They can be small informal settlements or they can house millions of people.