Taken from Wikipedia:

Leeroy Jenkins, sometimes mis-spelled Leroy Jenkins and often elongated with numerous additional letters, is an Internet phenomenon named for a character created by player Ben Schulz in Blizzard Entertainment’s popular MMORPG World of Warcraft. The character has become popular thanks to a machinima video of the game that circulated around the Internet. The phenomenon has since spread beyond the boundaries of the gaming community into other online and mainstream media.

Origin

The following is one among many rumors of the origin of Leeroy Jenkins. There are many versions of this tale, and no one but the creators of this phenomenon will ever know for sure what is true.

The video is of an actual event. It takes place in Upper Blackrock Spire (UBRS) in World of Warcraft, inside the Rookery room, one of the most notoriously difficult sections for newcomers to the game and responsible for countless wipes and other mishaps similar to the one depicted in the video.^[1] It opens with the guild members discussing an impending raid via audio teleconferencing, complete with regimented battle plan and statistical breakdown of their survival. The fastidiousness of their preparations is ruined by the sudden and unexpected actions of Leeroy Jenkins, who was away from his computer. After missing the entire conversation, Leeroy suddenly charges into the Rookery, yelling, “Let’s do this! LEEEROOOOOOYYY JEEEENNNNKIINNSS!” There is about a second of stunned silence from his companions, followed by carnage when they proceed to hastily attempt the attack (as Leeroy’s actions have now alerted the monsters). The attempted raid (and video) is filled with alot of confusion from the team to mount their original plan, as well as insults at Leeroy, with Leeroy calling out, “It’s not my fault”. The battle ends disastrously, with all the guild members lying dead on the floor, screaming at Leeroy for his brash decision. Leeroy’s only response is “At least I have chicken”, as he had spent the party’s planning time reheating chicken in his father’s kitchen. he now remains on Server Sargeras with the raid leader Happyjack.

Phenomenon

This Internet phenomenon started with the release of a video clip online to the World of Warcraft forums. The video was released by the World of Warcraft Alliance player guild PALS FOR LIFE on the Laughing Skull PvP realm.

The publication of an article in the August 2005 issue of PC Gamer UK, entitled “The Ballad of Leeroy Jenkins” created greater awareness. The article claims that the original video was designed as a negative commentary on the kind of “nerd-guilds” that meticulously and statistically plan out raids the way Leeroy’s guild was apparently doing. Leeroy is in fact the hero of the piece, acting against the geekiness of his guild.