MMORPG is shorthand for Massively Multiplayer Online Roleplaying Game. These are role-playing games where the game is played over the internet with thousands of other on-line players. Guild Wars is not a true MMORPG, however. While players will be able to talk with and meet thousands of other players in outposts, once they venture into the game's explorable areas, they will be playing in a closed space that includes only their party.
Common parlance has also added an additional defintion for this term: Mostly Men Online Role-Playing Girls. No one has the ability to prove the veracity of this statment, but as the internet culture tends to be dominated by the males of western culture it would stand to be true.