"Zerg" is probably one of the more common words that come up when you talk about large-scale PvP in MMOs. Other words synonymous with "zerg" are "swarm," "overkill," "mindless," "%#@#$!" and "what a bunch of lamers." Okay, the last two were expressions, but they're probably the most accurate.
When you call your enemy a "zerg", it almost always means they have more people than you, and if you're angrily calling them a "zerg," it probably means they're kicking your heiny. There are occasionally intelligently organized zergs, but most often they're mindless groups of players trying to overwhelm you by sheer numbers (not skill). They can be outclassed if you know what to do and this guide gives you the strategies to do it.
Zergs are common in WAR because the RvR-centric game is built for tens to hundreds of people to try kill each other out on the battlefield. Group sizes are small (6 people), however, you can combine up to four groups and make a warband (24 people). Oftentimes you will encounter several enemy warbands when they are trying to lock a zone.