Unlike general purpose maps, thematic maps are intended for one specific purpose: to communicate a particular topic or theme.
There are many different types of thematic maps. Each uses a different symbol to depict information. Four of the most common types are identified above.
A choropleth map is based on numerical data on some topic, with data available for enumeration districts (usually political units such as counties, states, or countries). The numerical values are used to place each of the districts into a particular category. An area symbol (usually a color or shade of gray) is then used to show which category each of the enumeration districts falls into.
This link takes you to a very nice program that will create choropleth maps of United States Bureau of the Census data. Highly recommended!
Dot maps employ dots to convey information about a distribution. A single dot may represent one occurence of something (an earthquake, perhaps) or more than one (a dot might represent 100,000 people, for example).
Graduated circle maps also use a point symbol, but the symbols have different sizes in proportion to some qualtity that occurs at that point. The populations of different cities are frequently depicted on graduated circle maps.
Isoline maps use line symbols to portray a continous distribution such as temperature or elevation. Isolines are lines that connect points of equal numeric value. One of the best-known types of isoline map is the contour map, which shows elevation above sea level.
Strictly speaking, cartograms are not really maps because they are not drawn to scale. Instead, areas on the cartogram are intentionally drawn larger or smaller in proportion to the phenomenon being shown. A cartogram of world population, for example, might show countries as being either larger or smaller in proportion to their populations.
