Most MMORPGs, such as World of Warcraft and Final Fantasy XIV, are based in fantasy and myth and involve magic and spells.
Some are based in space, where you must command a spacecraft or your own planet.
Some are even based on the real world, and with the invention of Google maps it may well be possible to have an MMORPG world which closely emulates the real world, perhaps even being able to visit your own home!
MUDs, or Multi-User Dungeons, were the first MMORPGs.
They are usually simple text based programs where the players use commands to control and interact with their character, the world, and other players.
Although simple 2D graphical versions and even 3D MUDs do exist. Similar to MUDs are browser based MMORPGs, like RuneScape, which are played entirely in the users browser.
They can be simple pages of text or complex 3D renderings and offer similar functionality of more developed MMORPGs, usually for free.
MMORPGs were virtually unknown years ago and now they are commonplace for most gamers.
In fact worldwide revenues for MMORPGs exceeded half a billion dollars in 2005, and Western revenues exceeded one billion USD in 2006.