For the record, graph theory is defined at Mathworld as "the mathematical study of the properties of the formal mathematical structures called graphs." Not particularly enlightening. However, go here and read about graphs. This write-up is accessible to all of you although you might want to skip the links to eigenvalues. These graphs are actually pretty neat. I did go to a seminar a couple of years ago that was an introduction to graph theory. We all got to try and work out the bridge problem (you'll have to go to the link to find out what that is).