"In assortative networks, well-connected nodes tend to join to other well-connected nodes, as in many social networks (i.e. friendship links in a school in the United States).
b, In Disassortative Networks, well-connected nodes join to a much larger number of less-well-connected nodes. This is typical of biological networks." (from http://www.nature.com/nature/journal/v453/n7191/fig_tab/453047a_F1.html)