mmorpg stands for Massively Multiplayer Online Role-Playing Game
mmorpg's are games where you and up to thousands of other people play in the same server (world) and you level up just like in diablo. There are so many differences between any normal mmorpg and diablo. An mmorpg is a lot more realistic than games like diablo or bulder's gate. It's basicly a step-up to normal RPG's. Hope I helped you understand a little better.