In a nutshell, moderating on DU3 is split up into different responsibilities. Juries take care of civility. Hosts keep forums and groups on topic. The Malicious Intruder Removal Team keeps out the freepers.

Introduction to Jury Service video:
http://www.democraticunderground.com/1240252

Information about joining the MIR Team:
http://www.democraticunderground.com/1013163

Information about Hosts can be found by clicking the "About this forum" or "About this group" button found at the top of every forum and group.

Full explanation of moderating system on DU3 (very long):
http://www.democraticunderground.com/?com=modsystem