Choropleth maps are themeatic maps that use color coding or shading to display data concentrations. A key is needed to determine map coding, as each map has a specific shading with relation to its own data.
Found - http://www.ers.usda.gov/AmberWaves/May07SpecialIssue/Findings/Charts/Findings_greene_fig01.gif
This blog is a map catalog in which I am displaying fifty different types of maps and a tid-bit of information that looks into the purpose of the map!