Choropleth Map

http://en.wikipedia.org/wiki/File:2004US_election_map.svg
A choropleth map (coming from the Greek words "area/region" and "multiply") is a thematic map, in which areas are shaded or patterned via certain color patterns. These represent the measurements of whatever variable is being displayed on the map (i.e, population density, per-capita income, etc.). The choropleth map is an easy way to visualize how measurements can vary across big - or small - geographic areas (i.e, the whole United States). It can also show how much a factor varies within a particular region, country, state, etc. Color progressions include single-hue, bi-polar, partial spectral, blended hue, value, and full spectral color progression.
The choropleth map shown above features the voter percentage difference in the 2004 presidential election.