The Lich King is the ruler of the Undead Armies. He is not a physical being however; he is a spirit-like entity that commands through telepathic messages to his commanding officers. His "spirit" resides within the Frozen Throne: basically a throne completely frozen within layers of ice, and perched on top of a huge mountain of ice and rock.
In the storyline, a former paladin named Arthas becomes power-hungry and searches for Frostmourne, a sword and a powerful artifact that contains the essence of the Lich King. Once having obtained the sword, Arthas becomes possessed to do the Lich Kings biddings, and wants more power.
Soon, after Arthas is able to squelch the opposing forces, he travels to the Frozen Throne as dictated by the Lich King, breaks the ice away from the throne with Frostmourne, and puts on the crown of the Lich King; The Lich King has returned.
Footman 1: Whoa, what are those varmints travelin' this way?
Footman 2: They call 'em the uh... Undead I reckon.
Footman 1: The what? That's crazy talk. Noone can bring the dead back to life silly.
Footman 2: No seriously! I heard about this one bloke that has this incredible mystical power to raise the armies of the undead.
Footman 1: Well, what's his name dude?
Footman 2: Uhh.... It was.. I think it was Ronald McDonald or something like that.
LICH KING: I AM THE LICH KING, RULER OF THE UNDEAD ARMIES. SURRENDER TO THE WRATH OF THE SCOURGE! YOU WILL NOT SURVIVE.
Footman 1: Oh wait, I remember who he is. He's Bob the Builder.
Footman 2: No.. it's not him. It was Ronald something..
Footman 1: Ronald Reagan?
Footman 2: Yah, I think so. Yah! Ronald Reagan that's who.
Footman 1: Well damn him to hell I say.
Footman 2: Say, I'm kinda hungry. Wanna go get some McDonalds?
Footman 1: Nah McDonalds is a lardbucket. Tis for small children and fatties.
Footman 2: Well you're not looking very fit either.
Footman 1: Well I'm trying to work it off. 'S why I joined the army. It's a good workout.
Footman 2: Aren't you worried that you might die any second?
Footman 1: Not really, I mean, there's really nothing for me besides this. I have no children, no friends, and my wife is.. pretty much a bitch. Nope, I'm fine with the army.
Footman 2: I just joined cuz I needed some money. Completely broke dude. I wouldn't be standing here in front of you if it wasn't for the dollar menus.
Footman 1: Amen to dollar menus bro. I also kinda like that restaurant off Fifth Avenue, W-
LICH KING: I HAVE GIVEN YOU A WARNING. NOW YOU SHALL FEEL THE FULL FURY OF THE UNDEAD SCOURGE. YOUR HOMES AND SETTLEMENTS SHALL BE DESTROYED. YOUR WIVES AND CHILDREN SHALL BE MERCILESSLY MURDERED AND FED TO OUR GHOULS. YOUR WORLD AS YOU KNOW IT WILL CHANGE. THERE IS A NEW POWER IN-
Footman 1: HEY CAN YOU KINDA.. I DUNNO.. STFU? I'M TRYING TO HAVE A DECENT CONVERSATION HERE.. GAH! Cmon dude, let's go to Borders. At least it'll be quiet there. SO RUDE!