“Do they fight over territory, or some philosophical difference, or is the membership by ethnicity, or what?”
All the above depending on which gangs are involved. Usually it boils down to money (from drugs, prostitution, gun running), territory (which is all about money and street cred), and bragging rights (who’s the biggest, baddest, most feared, etc.). These gang rivalries are everywhere you find the biker gangs - even New Zealand: http://en.wikipedia.org/wiki/Gangs_in_New_Zealand
Thanks. One of the articles linked in this thread said these sames biker gangs involved in the Waco fight also operate in some European nations and have periodic violent clashes.