Urban Dictionary defines zerging as "To defeat an opponent using many waves of expendable lives.". In the BZPC term, it basically means to swarm something.
A Vote Zerg is when many amounts of people vote for the same person in a very short amount of time (seconds to a few hours) or at least if all their votes get counted at the same time. HOWEVER, the zerg has to be planned before hand, or otherwise, it's just a coincidence a lot of people voted for the same person.
A Noob Zerg is when a large amount of noobs log in at one time, usually the noobs online are Mini, Jedi, Dave, Blackout,and Shadrahk.