Frollard is quite right. Another complication for multi-layer boards is accessibility. With a single layer board, the conductive paths ("traces") are on the surface. If there is an error on the board you can go in with a knife to cut the trace and you can solder wire to the surface traces to patch it to another location. With a multi-layer board cut and patch is much harder. Combine that with the cost and you have to be sure to get the multi-layer board right on the first go around. You can think of the multi-layer board like some complex highway interchanges called stack interchanges (here is a Wikipedia link with lots of pictures). If you have a couple highways coming together you have to stack bridges up one on the other so that everything can pass through the area.