Here is a tutorial from Maxim on common-mode signals.
To understand how common-mode signals are created and then suppressed, you should first understand the interaction of shields and grounds in common cable configurations. The following discussion defines a common-mode signal, reviews the common cable configurations, considers shielded vs. unshielded cables, and describes typical grounding practices. The article discusses methods whereby common-mode signals are created and rejected.
The primary focus of this discussion is on RS-485/RS-422 cables and signals, but the discussion also applies to telephone, audio, video, and computer-network signals.