How can and do a MMO have exspansions?

[Deimos]

How can a MMO be a whole mmo, but have different expansions that let people play in more places and more levels?

it sounds kinda complex to me. what if someone signs up for a MMO, then a expansion comes? does his current version gets upgraded, or does he have to get the expansion?

it almost sounds like the player is playing a single player RPG with sequels, but has online game play. is that possible?

hurrrr gee maybe the expansion adds content and areas that only another player with the explanion can access kinda like how you gotta be a member on runescape to access the member features but your character can still co exist with them in the game world hurrrr durrrr nurrrrrr