For a more complete list of zones, see or Game Information by Zone.
The world of Azeroth is divided up into certain distinct areas referred to as Zones. Each zone typically has a single type of landscape, and its own unique set of cultures, peoples, and concerns. There is no consistent size to a zone, and they vary from rather small (Deadwind Pass) to quite large. (The Barrens)
Most zones are designated with a specific range of levels that are the best time to visit that area. Certain zones are better for Horde or for Alliance, and some are even designated as Horde or Alliance, while the other zones are simply labeled Contested.