A large group of players who usually outnumber their opponents. This typically also involves cloning and attacking over and over, often without healing or buffing first, in an attempt to win via sheer numbers. This term originated from the game Starcraft, in which the Zerg race did just that.
A geek-speak add-on to the end of words, usually increasing the initial value of the word. For example, someone can get pwn3d, but when they get pwnz0red, it's seemingly more humiliating.