The clip begins with ten players, including Leeroy the Paladin, planning a raid on part of a dragon-filled dungeon. The players are heard discussing tactics (Abduhl notes, "We have a 32.33, repeating, of course, percent chance of survival", another remarks that it's better than they usually do.) before they enter, however, Leeroy remains quiet.
In real life, Ben Schulz was away from his keyboard and was not paying attention to the discussion. Just as the team begins discussing the final plan for its assault, Leeroy suddenly springs to life, shouting his battle cry of "Alright thumbs up, let's do this! LEEEROY JENNNNNKINS!".
He then charges fearlessly (or perhaps idiotically) into battle, to the complete and utter incredulity of his teammates. Attempting to rescue their comrade, the team charges in and attempts to execute their plan but they are quickly overwhelmed. At the end, Leeroy is chided by his allies for leading them to their deaths, and he responds with the famous line "At least I got chicken." Referring to his activity away from keyboard.
Leeroy Jenkins got so famous that he was even featured as one of the questions in Jeopardy!
Hit play or go to Link [YouTube] to see how Leeroy got everyone at his WoW raiding party slaughtered - Thanks Allan Sorensen!