RealmServer

From WCell

A RealmServer (or World Server) can run one Realm (or one World) in the WoW universe. Each Realm has to register with an AuthServer (or Authentication Server) via IPC in order to be accessible by users.
The users log into the AuthServer and after selecting one of the available Realms, they will connect to the RealmServer where they can select any of their Characters to login with and start playing.

Regions

Each Realm usually consists of all major Regions: Kalimdor, Eastern Kingdoms, Outland, Northrend and a variable amount of Instances and Battlegrounds.