Re-Topology Modeling
Re-topology means creating a new mesh surface from scratch, that conforms to an existing "messy" mesh, often created via sculpting. Messy meaning that it either has too much detail or has a topology that's not the best for texturing, shading, or animation. The details from the "messy" mesh are transferred to the new re-topologized mesh by baking a DisplacementMap or NormalMap. The new mesh can either be used directly in-game, or it can be used as the control cage for a subdivision surface.
Tutorials
Optimisation workflow tutorial - by Bram "Peris" Eulaers
Using Meshlab to optimize a Mudbox environment sculpt for import into 3ds Max.
Retopo workflow - a Polycount thread
Short discussion of re-topo methods for a ZBrush character model.
Retopology in Maya - a Polycount thread
Longer discussion of re-topo in Maya and other apps, like Topogun.
ZBrush Decimation Master With HD Sculpting - by Nick Zuccarello
In-depth video tutorial with voice-over.
Resurfacing A Mesh In Maya - by Nick Zuccarello
In-depth video tutorial with voice-over.
Creating Topology in Topogun - by Joe "stoofoo" Pikop
In-depth video tutorial with voice-over.
Re-Topologize a Game-Ready Alien Head in Blender - by Jonathan Williamson
In-depth video tutorial with voice-over.