In heavy timber framing you use heavy timbers, as opposed to "stick framing" where you use 2x4s and such.
Another important difference is that in the best timber frames, each framing member is cut to fit into the other pieces, or "joined," like a 3D puzzle, in a way that resists the structural forces of the building. The joints are usually started with a saw and then finished very precisely with a razor-sharp framing chisel. The joinery is pegged with wooden dowels, so there are no metal connectors whatsoever in the frame of your house.
Here are a couple of fun timber framing links for you guys to look at:
http://foxmaple.com/ Fox Maple school of traditional building techniques
http://ecositacr.com/ A timber framing company in Costa Rica.
http://www.barrtools.com/ The best framing chisels. My understanding is that this gentleman was trained as a Japanese-style swordsmith...I've got a couple and they hold an edge like nobody's business.
http://www.jcastle.info/ Heavy timber frames are super-strong and long lasting. I think these castles are some of the coolest buildings on Earth.
I like timber frames!