Isopleth maps can be compared to choropleth maps.  They both depict a variable with a line that cannot be represented by a single point.  It is used to convey information across a large area.  This being said all the different types of isopleths such as, isobar maps, isotach maps, isohyet maps, etc are all also isopleth maps.  This particular example of both an Isopleth and Choropleth map is showing amounts of acid rain across the eastern half of the United States.

Isopleth of Acid Rain