1. *Bullies* - who repeatedly make some sort of attack on someone who is (for some reason or another) unable to defend against it (an *asymmetric* relationship).
2. *Passive victims* - who usually don't provoke the bully, they might just be different - or weak - or handicapped - or smart - or not something...
3. *Active Victims* - who tend to be very good at getting under someone's skin - either by the way they say things (perhaps they have a great way to humiliate someone verbally) - but they usually end up seen as the ultimate victim. If you trace things back, active victims look a lot like bullies, but in a different way. They often blame others with a type of rapid revisionist history of events.
4. *Bystanders* - they tend to *normalize* what is accepted in the social setting - so what might be considered bullying by one group, might be considered *normal* by another - which is one reason why you can talk to a teenager all day about not bullying, and they have one view of what that means and sending a *mean IM* probably isn't going to be it, unless that child identifies themselves themselves as the victim.
On a related note - however kids define bullying, more than half say they have been both victims and bullies in different situations, and like all models - the *4 groups* listed above is just a handy way to help some get a handle on the way many situations play out.