Wizards and Warlocks - What is the difference?

[tangawarra1]

No: HPFGUIDX 35124

I noticed last night that in CoS, When Ron, Fred and George rescue 
Harry from the Dursleys, Ron talks about his Dad's work at the 
Ministry of Magic:  "its only dad and an old warlock named Perkins 
there" (paraphrased. CoS). 

I also think I remember a reference to an International Confederacy 
of Warlocks in either "Quidditch Through the Ages" or "Magical Beasts 
and Where to Find Them".

Does anyone else have any idea what the difference between a warlock 
and wizard might be?  Perhaps there are levels beyond O.W.L. and 
N.E.W.T that might confer a different title?  

Rach