I love the lore of Warcraft. It's too expansive to adequately describe all of it, but my favorite section of the lore has to do with Draenor. Draenor is a separate planet from Azeroth, the main world of the story. It is the homeworld of the Orcs, who comprise the majority of the current day Horde. It is the homestead of the Draenei. It is also the staging ground of the Burning Legion's assault on Azeroth. The Burning Legion is pretty much the main antagonistic force in World of Warcraft, and is responsible for everything from the death of the World Tree to the rise of the Lich King. Sargeras, corrupted Titan (creators of the universe) and leader of the Legion, has his hands in almost everything bad that has happened to Azeroth.
In its first iteration in World of Warcraft, Draenor was known as Outland. It was largely taken up by scorched wastelands where the Burning Legion had invaded and destroyed it, though there were a few undesecrated places. In the current expansion (Warlords of Draenor), players have been taken back in time to an alternate Draenor, not yet ravaged by the Legion. Here, they have been fighting the old Orcish leaders in order to stop a new invasion of Azeroth. Basically, Draenor is as much of a central location to the story of Warcraft as Azeroth itself.