Stands for "Massively Multiplayer Online..." So MMORPG is an online roleplaying game, MMOG is just a generic online game, and so on. "Massively" means it can cope with hundreds of players rather than two.

In particular, I plan a virtual copy of the entire disc, all 800,000,000 square kilometres of it, precisely duplicated on your computer in 3D down to the nearest blade of grass.