Choropleth [ˈkɔːrəˌplɛθ] : A map with geographical areas colored in relation to a data variable.
It is used to display patterns or variation of the variable in different districts of a region. For example it’s popular to show different voting behaviour between the electoral districts of a country.
The type of data determines what color scale to pick for the map. Sequential data is often displayed with a color gradient from light to dark. Maps showing diverging data, for example the derivation from a specific comparative value, get a scale with two color gradients and a fixed color for the comparative value. If your data is qualitative, every category gets its own individual color in the map. A great tool to find a good color scale matching your data is colorbrewer.
A common mistake when building choropleths is to colorize absolute values instead of normalized ones, which will falsify the visualization. Furthermore, keep in mind that the geographical size of areas might not correlate with their importance. This may lead to situations, where large but low populated areas might be perceived as more important than they actually are.