A Gameworld is a specific implementation of a world described in literature or other reference materials. Many servers provide a module which is based upon a particular game world. Each game world has it's own flavor and theme. Many game worlds may be implemented on the different servers and will perhaps have differing themes. For example, one Dungeon Master's idea of how the Forgotten Realms should be run may be different than anothers. Each may develop different modules to run on their own servers that portray the same place and time in particularly different lights. Even if each implementation was the same, each one is considered to be it's own, separate, game world.
The term "Gameworld" is often used coloquially for "realm", or "game setting". A particular "world" is an implementation of some described setting. A "realm" is the "type" or "class" of that world.