In 1994, a small, California based software company created a game called Warcraft: Orcs and Humans. They were never heard from again. Unless you count two sequels, their expansions, spinoffs, and the most popular MMO of all time.
This is where the Warcraft universe really started the flesh out and become what it is today. It introduced many of the factions and characters that continue to drive the story to this day. The night elves and undead were introduced, as well as the Forsaken and the Burning Legion. The Forsaken are undead who escaped the control of the Lich King who commands the undead Scourge (ie. the bad guys). The Burning Legion represents the ultimate evil in the world (...of Warcraft). They are led by the fallen Titan, Sargeras, and are bent on the destruction of all life in the universe.
We also meet Arthas, the whiny bitch this whole saga revolves around. We meet him as Prince Arthas, heir to the human kingdom of Lordaeron. His people start dying to a mysterious plague that turns its victims into mindless undead. He sells his soul for a way to stop it, and (since we all know how well that usually goes) ends up as a death knight and the Lich King's right hand man. Then *spoiler alert* he becomes the Lich King. That's about as big a spoiler at this point as finding out that Darth Vader is, in fact, Luke's father, but that's beside the point. Arthas drives most of the story of the Warcraft universe until his arc wrapped up in the most recent WoW expansion: Wrath of the Lich King.
World of Warcraft puts the player into a starring role in the ever changing, ever expanding Warcraft universe. As a crafty adventurer, you battle everything from animals to orcs to dragons to demigods to the dread menace known only as Hogger. For the first time, players get a role in shaping the future of the world. They get to meet their favorite characters from lore... and kill them! Thus far the game has taken us from the familiar confines of Azeroth to the distant lands of Kalimdor, the crumbling world of Dreanor, and the frozen continent of Northrend. Many of the story arcs that began in Warcraft 3, have been wrapped up with events that have taken place since the start of WoW. There are far too many to go into in this small space, but rest assured, the body count was sufficiently high.