A “red shirt” according to Star Trek conventions is a Security guy whose sole purpose for existence is to die, just so the main characters never get killed but are still surrounded by death and danger.
At the recent Blizzcon 2010, a red shirt was the one doing all the pwnage though! Check out this video of this fan asking a really tough question about some Warcraft lore and totally stumping the Blizzard guys.
A lot of the YouTube comments berate this guy for being a retard, but I say leave him alone! He might not be getting laid anytime soon, but he knows his shit!