I know about maintenance but sometimes I will get kicked out of WoW and won't be able to log back in for a while because the "world server is down"
What does it mean exactly?
Basically, it means that Northrend, Outland, Eastern Kingdoms, Kalimdor, or any or all of the above are broken right now. If you're inside an instance, you'll still be fine, but you'll get disconnected upon leaving. This is usually a temporary thing, sometimes from server overload, sometimes from a misapplied hotfix.
What you see as a single Realm is actually a collection of servers that do separate things. Off the top of my head some functions that are done separately are user authentication (logging in), instance servers, Battleground servers and of course the World server that ties all of these together. If I remember correctly some of these servers (or clusters of servers) are all shared across your Battlegroup which is why you see bunches of servers affected by bugs/maintenance at the same time. This sharing is what allows cross-realm Battlegrounds and soon, hopefully, cross-realm dungeon/instance grouping.
Corrections welcomed. :)
It means that the server governing the continent which your character currently is on is not working properly. If it didn't stop you from logging in then you would simply be unable to load any of the graphics for your current location.
lol i was wondering that to wtf does world server is down mean haha ty....do u know how long it will take to be "world serever is up" lol.
Basically, complicated distributed applications, like WoW, are broken into multiple services. These services could be varied like, LoginServer, CharacterServer, InstanceServer and, of course, WorldServer.
Basically, what that error means is that the CoreServer has started, and immediately loads LoginServer, and CharacterServer. However, probably due to the time it takes for the WorldServer to read and configure the last known state of the world, it can take a very long time for this to finalise.
On top of this, you could also get a World Server is Down error when, for some reason, you and the world server are de-synced from each other. Maybe the world server is lagged out and it thinks you are from the future.
It's also possible for different parts of the world to be down. Eastern Kingdoms can be up, while Kalimdor is down. That would suggest that each section of the world are subordinate services to the world server.