My brief summary of "Leroy Jenkins":
A party in World of Warcraft is sitting waiting for Leroy to come back from AFK and discussing on Teamspeak what to do next. Someone points out that Leroy wanted to go into a particular room to get an item he wanted. Evidently, the room is pretty tough to fight through and the group leader is less than thrilled about going in. They start discussing tactics for going in and go to great length about specific roles. In the middle of the conversation, Leroy stands up, screams "Leeeeerooooooy Jennnnnkiiiiiiins!" and runs into said room. A few seconds and some bad language later, they are all dead, and angry at Leroy.
I think anyone who has played any type of cooperative online game has played with someone like this. "Leroy" has become the online slang for someone who charges in not caring how much he/she aggros.
This has reached such magnitude of MMO Lore that Anet added an NPC in the Grenth's Footprint/Sorrows Furnace update named Kilroy Stoneskin. When you help him with his quest, he uses a shout skill called "Killllllroooooy Stoooonskiiin!" and then charges into groups of stone summit for a fight. The quest is to help him and keep him alive.