IMHO "bigamy" is having two wifes, while "polygamy" is having more than one in general (thus includes bigamy). But you kids shouldn't be asking about this!
In most Western cultures -and I'm sorry I have to crush your wildest dreams- monogamy is the standard.
For the record: the name of the most downloaded MSN Messenger patch in the world was initiated by
Asdfuae, who's probably more familiar with the concept because of his origin. The first "polygamy" patch I got my hands on was created by
GlObAl/
PAIN (back in the MSN 3.x days), but I guess my "Simultaneous Sessions" denomination didn't sound as catchy back then as polygamy