Univariate choropleth

This is an example of a univariate choropleth map; the only measurement is of population density (people/sq. mile) in the desired area.

http://gis.esri.com/library/userconf/proc96/TO50/PAP028/P02819.GIF

Continuously variable proportional circle

This is an example of a continuously variable proportional circle; the symbols depict the ratio of Internet Users in 2004 Europe.
http://www.geog.ucsb.edu/~jeff/gis/proportional_symbols_files/map2.jpg

Classed choropleth

This map is a classed choropleth, but it is not areally averaged or standardized in any way. The intervals are the unit being measured.

https://courseware.e-education.psu.edu/courses/geog482/graphics/citationexample.jpg

Isopleth

An isopleth is a feature of meteorological charts, connecting points which have an equal value of some variable at a given time and spatial area. The particular variables shown on a given chart may include values such as pressure, temperature, wind speed, etc. They are used to assist in visualizing the general features of a meteorological "field" (http://en.wikipedia.org/wiki/Isopleths)

http://www.krisweb.com/kristenmile/krisdb/html/krisweb/maps/ten_rain_map.gif

Isohyets

Isohyets are lines of equal precipitation, depicted in millimeters.
http://www.ecohydrology.uwa.edu.au/?f=192830

Isotachs

This map shows isotachs, lines on a given surface connecting points with equal wind speed, together with isobars, the line of equal atmospheric pressure.

http://www.underthemeso.com/blog/wp-content/uploads/entry_20070223_1.gif

Isobars

Isobars are lines of equal air pressure; winds are stronger where lines are closest together.
http://z.about.com/d/german/1/0/1/J/aWetteurfronts.jpg