A cartogram is a map in which some thematic mapping variable, (such as travel time or Gross National Product), is substituted for land area. The geometry or space of the map is distorted in order to convey the information of this alternate variable. There are two main types of cartograms: area and distance cartograms.
An area cartogram is sometimes referred to as a value-by-area map or an isodemographic map, the latter particularly for a population cartogram, which illustrates the relative sizes of the populations of the countries of the world by scaling the area of each country in proportion to its population; the shape and relative location of each country is retained to as large an extent as possible, but inevitably a large amount of distortion results
A distance cartogram may also be called a central-point cartogram. This form is typically used to show relative travel times and directions from vertices in a network.
http://mapmaker.rutgers.edu/355/cartogram_ValueByArea2.jpg
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