the image in the bottom left shows some of the modeling that contributed to that desk. The lowpoly has just a flat face in that spot. RH_hipolyWires_011210 by sprunghunt, on Flickr
The one on the left is based on a fameous person. Recognize him? It is a caricature. The one on the right is from imagination, and more of a lose sketch, sloppy style.
A and B are just inputs, input slots are always on the right side of the nodes and output slots are on the left side. http://udn.epicgames.com/Three/MaterialsCompendium.html