Floating high in the sky over the Crystalsong Forest in the center of Northrend is the magical city of Dalaran.
This city was originally located in the Alterac Mountains, relatively close to Arthas Menethil's homeland of Lordaeron. It is the home of the powerful mages of the Kirin'Tor. When Arthas began his descent into madness, this was one of the cities that he laid siege to. After its destruction, the Kirin'Tor placed a giant bubble around it for protection, and began the long process of rebuilding, which lasted for all of the original WoW and Burning Crusade time periods.
When it was decided that it was time to take Arthas out, the barrier was removed, and the rebuilt city was transported to its current position in Northrend, to serve as a forward base for anybody who was taking the fight to Arthas.
Like Shattrath in the Burning Crusade, Dalaran served as the home base for characters of both factions, when they weren't questing, doing dungeons or raiding. This city is designated as a sanctuary, so there is no combat permitted within its boundaries, but there are clearly specified Horde and Alliance-only areas, and players not of the corresponding faction will be transported out if they try to access them.
In the center of the city is the Violet Citadel, where Rhonin, the leader of the Kirin'Tor and his advisors convene. You can hop a portal to Tanaris' Caverns of Time from here, and it is also the place to be as you turn in the last quest in a chain that was implemented late in the Wrath of the Lich King expansion, that has you reforge a broken epic sword.
The Violet Hold is a prison for some of the most dangerous enemies of the Kirin'Tor, and serves as a mid-70's dungeon, with players attempting to stop a jailbreak.
Meanwhile, the sewers of Dalaran are a haven for the less savory characters. There is a market for rogues, and even an underground arena, where players can duel, and participate in matches.
Also of note in the sewers is this Level 80 rat, and his four baby turtles.