TBH, it’s not necessarily a java newless clubie question, and may instead be an MMOG newbie question (which we dont have a forum for).
ironbeatle: if you are asking because you don’t know how to write a game with a data model where data is stored on a back-end, and is rendered on a front-end by clients on player’s PC’s, where the data actually lives on a server … then you really need to go and read up on how multiplayer gaming and networking in general works. The stuff you read is the same irrespective of language (networking is one of the great-levellers for programming-languages -it really doesn’t matter what the heck language you’re writing different bits in).
Go to gamedev.net and read their (many) networked-game articles.
If you’re asking because you DO know all that, but want to know what’s special about runescape etc, the simple answer is “multiple processors” - this may be in separate physical machines (multiple servers) or in one machine (multiple threads).
Unless your server runs ONE thread only, that does EVEYRTHING (i.e. the opposite of thread-per-player), you should start learning transactions, ACID rules, etc, and how to do transactional operations on databases. Google time.