C4D's are just something made in Cinema4D. Here's an example of one.
C4D
They are generally very abstract and flow-y. They are most often used in sig making. You would copy and paste it in a new layer over your sig, move it around so it flows with your sig, then mess with the blend modes and opacity. For more on sig making, check out these tuts.
The Ultimate Guide to Tags
A signature tutorial by DarkShock
And another sig tutorial, by Yy10