Density

Density in SimCity is the key to high populations and high profit, as well as tons of traffic. High Density is what makes buildings grow into skyscrapers and low Density keeps them in the form of single houses and storefronts. To increase Density you have to increase the area's Happiness, as well as have roads that can support higher densities and that are spaced far enough apart for the higher density building's footprint. Note: Density and Wealth are completely different things and are not effected by one another, you can have Low Wealth/High Density apartment buildings and High wealth/Low Density mansions. This goes for commercial and industrial zones as well.

Low Density

Low Density is where all areas in your city will start. In these areas you will see single houses or trailers in residential areas, single stores in commercial areas, and small factories in industrial areas.

Medium Density

You will likely start finding these pretty quickly in the heart of your city once you have all of your basic services established. These are your smaller apartment buildings, a larger factory, and multi-story commercial buildings.

High Density

Once your city has been well established and running smoothly for a while you may start to notice High Density buildings spring up in the heart of your budding metropolis. For Residential buildings these will be skyscraper apartment buildings, commercial buildings turn into skyscraper office buildings, and your Industrial zones will become giant factories.