Northrend is northern most continent of Azeroth, assumed to be sitting on the North Pole of the Planet. It is a place of ice and dread, the birth place of the Scourge and the seat of power to the Lich King, one of the most powerful and evil figures in all of Warcraft Lore. Northrend will be the main focus of the Wrath of the Lich King expansion.
The zone of Northrend include: Azjol-Nerub, Borean Tundra, Crystalsong Forest, Dalaran,The Dragonblight, Grizzly Hills, Howling Fjord, Icecrown Glacier, Lake Wintergrasp, Sholazar Basin, The Storm Peaks, Zul'drak.
From a gaming point of view, Northrend will be larger (both in number of zone and scope) than Outland was. The biggest zone of Northrend is also said to be much bigger
Northrend was once the northern part of Kalimdor, but following the Sundering it shattered off from the main land and drifted away from the temperate climate into the polar area. The sudden change in temperature killed off many of the surviving wildlife, ill adapted to their new northern climate.
The strong and tenacious race of Spider-Men, the Aqir, managed to survive and even thrive on the new frozen continent. They however, did so by living underground, away from the elements. The Aqir evolved into the Nerubian of the millenium, a people of shadowy and mysterious scholars and sages. While a few humans and trolls managed to survive on the continent as well, their presence was dwarfed by that of the Nerubian empire.
Of note is the The Dragonblight, the final resting place of many great dragons. This boneyard of draconic origin was established on Northrend, the frozen and near deserted land seeming a peaceful and isolated enough location. To further ensure that their dead would remain undisturbed, Malygos, the Blue Aspect, settled on the continent, keeping a watchful eyes on his fallen kin.