I’m somewhat new to geocoding. One of the first things you might be interested in, if you have geographic data, is an indicator as to whether a certain address, zip code, or county is urban or rural. This is actually quite a complex topic. This paper outlines some of the basic systems to classifying a location as urban, rural, or something in between (e.g., suburban).
Susan A. Hall, Jay S. Kaufman, and Thomas C. Ricketts. Defining Urban and Rural Areas in U.S. Epidemiologic Studies. J Urban Health. 2006 Mar; 83(2): 162–175. doi: 10.1007/s11524-005-9016-3. Available in html format.
You can find an earlier version of this page on my blog.