World of Warcraft is a game that is sufficiently complicated to require its own terminology for a lot of things. "Raid" is one of such terms.
A "Raid" is a dungeon designed for a group of players numbering 10 or greater. These typically are complex and require a fair amount of coordination from the group. The rewards match the effort.
This evening I will be participating in a Raid for the first time. My guild is attempting to enter a powerful wizard's tower and delve into its mysteries. This guild was founded by an awesome married couple I met in-game and mentored somewhat, and through their hard work attracted other intelligent people. Our chance of death is staggeringly high.
Raiding is a large section of the game, and something I have heard much about but never experienced myself. I'll likely bring my feelings as to the subject here later.
I'd originally intended on delving into some theory concerning the group roles of WoW, but it came out incredibly dry and boring (even for me). You are all spared.