Bivariate Choropleth Maps, as the name implies, depict the distribution or occurrence of a two variables across a map and how they are found together. It is exceedingly hard to find some of these not through someone else's blog when one looks for them.
http://www.geog.ucsb.edu/~jeff/gis/choropleth_maps_files/election04_vs_crime03b.jpg
Univariate Choropleth Maps are concerned with the occurrence of distribution of a single variable across the depicted area. This distribution of votes by county takes only one variable into account: voting.
http://kevinschulke.files.wordpress.com/2008/11/2008_election_map.jpg
Unstandardized Choropleth Maps use raw data as their basis of depiction. No other mitigating factors are accounted or adjusted for.
http://cber.utk.edu/census/2k/map2.gif
Standardized Choropleth Maps use data that have been averaged or otherwise manipulated so that one is no longer dealing with the raw numbers. Population per square mile by county, for example, shows how theses numbers don no account for county size of other factors.
http://pubs.usgs.gov/ha/ha730/ch_c/jpeg/C007.jpeg
Unclassed Choropleth Maps use continuously graded scales to show information. There is a greater range of possibility for shade or color, and that shade or color will only exactly coincide when the data exactly coincides
http://www.esds.ac.uk/international/images/uchoropleth.gif.
