A picture may be "worth a thousand words" but when using diagrams for professional or scholarly purposes technical descriptions can be critical. An image without a thorough explanation increases the risk of the reader failing to recognize the importance of the concept or making false assumptions of the image's meaning.
The image above which was produced by Ernest Burgess in 1925 has since become known in urban geography as the Concentric Zone Model. The model which was devised to show residential differentiation and neighborhood change of 1920s Chicago is widely recognized in the field as one of the most influential models of urban structure.
The theory behind the model is that as a city grows in population so does economic competition, division of labor, and the complementary ecological processes of centralization and decentralization. While centralization occurs as a result of agglomeration economies (the economic advantages gained by similar companies clustering together among necessary service, financial, and utility services), decentralization occurs as other economic functions move toward the periphery.
Other Sources: Knox, Paul L., and Linda McCarthy. Urbanization: an Introduction to Urban Geography. Upper Saddle River, NJ: Pearson Prentice Hall, 2005. Print.