The world server is the actual working horse, its main purpose is to host a World Server Extension (WSE). A World Server also provides all the internal infrastructure for communicating with game clients, managing messages and priorities, deserializing and serializing messages to and from clients, persisting the base of Player object instances, flood protection and a lot of internal details and housekeeping.
Note that a World Server without a WSE can not exist! All the game specific logic is implemented inside a WSE. Technically speaking a WSE is a .NET assembly, a DLL loaded in the World Servers application domain. The WSE has to implement a set of interfaces which are periodically accessed by the turn manager.
The turn manager runs on a user defined frequency (typically 10-20 turns per second). During turn execution all user actions are queued in a second queue, once the turn is completed the queues are swapped and the turn manager starts a new turn processing messages from the full queue while the game clients are still sending new game requests uninterrupted into the second queue. Once the turn is completed the queues are swapped again and a new iteration starts until the World Server is shut down.
During turn execution all valid player requests (messages) ultimately get passed inside the WSE where the game logic is processed, therefore the WSE is the most essential part of the whole system. Unlike other similar frameworks the messages are transient for the Borg. Messages get validated and clients can get disconnected (or kicked or banned) if some rules apply or due flood protection but in the end every game related decision is made by the WSE. Same applies for the game logic, there are absolutely no restriction in what a WSE can do and what is processed in the World Server. The World server has absolutely no game logic. Though some common data structures like the Player and Character objects containing most common attributes (health, mana, experience, etc.) are provided, the WSE can choose to use them as is, ignore them completely or use them for something different.
You could implement a multiplayer Tetris game or a highly sophisticated collaboration network with Borg if you wish so.
The following diagram shows the main world server components:
