Everyone except China does play in the same game world on the same server. If CCP (Eve devs) had their way, China would play on that same server too.
But real life politik trumps video games, so that can't happen for now. Instead, the game is licenced to a Chinese company who runs their own instance for Chinese players so they can at least play the game - even if it's not with the rest of the world.
I wonder whether this was a commercial, cultural or geo-political decision. Does the CCP want to neutralise reactionary activity in Eve Online by controlling the server? Is there some political flavour to any of these alliances?
Some Chinese play on the world server but it's actually more or less illegal (wrt Chinese law) for them to do so...
Some years ago there was also an hilarious "invasion" of the Chinese servers by euro/US people, with patching the Chinese client to put it in English and using some stolen Chinese citizens national ID numbers in order to be able to subscribe... See http://failheap-challenge.com/showthread.php?1733-Serenity-C...
But real life politik trumps video games, so that can't happen for now. Instead, the game is licenced to a Chinese company who runs their own instance for Chinese players so they can at least play the game - even if it's not with the rest of the world.