Ground Loop Question

Just curious about this....

What exactly is a ground loop and why does it affect the stereo like it does? What exactly is happening to cause this? I'm a little rusty on electrical but i know the basics and terms. I would assume a ground loop is actually where a ground gets fed back into itself and maybe oscilates and causes the hum? or am i totally reaching here?

My buddy's ol man has a pretty decent mid-level system that is PLAGUED by this Hum. He has a MONSTER 3600 stage 3 power filter and the hum is still there.

Better yet, when he turns on the lights the hum gets WAY worse. (not a dimmer switch either) Normally i would attribute this to bad power, but shouldnt the stage 3 filter be clearing this? Does this sound like it could be a ground loop?

What are some common causes of ground loops and some common fixes?