Zerg is a term derived from the game Starcraft in reference to the tactics applied by the namesakes race. Zerg in a MMO is used to represent a mass amount of players on one team drastically outweighing the amount of players on the opposing team. The term is normally used as Zerg Rush which is a tactic where one team will send a mass of players to a particular spot with few enemies as quickly as possible to apply an advantage in numbers to a choke point or objective.
Zerg can also be applied to a tactic where players fighting a losing fight continue to press the issue by rushing into combat after respawning. This must be done while at least one member of the team is still in combat. The result is a trickling stream of players re-engaging into the fight prolonging it until a potential recovery is made or the fight is won/lost.