Understanding how subdivision surfaces work is the foundation of creating good looking models using Cinema 4D's polygon modeling tools. There are a number of fantastic resources available online - I'll do my best to keep this page current, but if you see something that isn't listed, please let me know!
Shepperd Oneill's Sub-D modeling tutorial series is a must-watch.
The following animated GIFs were discovered on http://www.pixelandpoly.com/reference.html
Head Modeling by Bay Raitt