As mentioned in MMO What? post, there are a plethora of genres or sub-genres in the industry. Here is a breakdown for each one. Some are acronyms while others serve as abbreviations. A few of these categories will grow well, but some like MMORTS will have a hard time due to the complexity of the games.

If you know of any more that exist but are not listed below, please let us know in the comments.

MMORPG = Massively Multiplayer Online Rolplaying Game
The most popular genre in the industry by far.

MMOFPS = Massively Multiplayer Online First Person Shooter
The second most popular type of free MMO. Gradually becoming more common with new games such as Wolfteam. MMOTPS is a variation of this to describe massive third person shooters.

MMORTS = Massively Multiplayer Online Real-Time Strategy
Currently very scarce in both pay and free versions.

MMOShMUP = Massively Multiplayer Online Shoot Em Up
Just one game known as Valkyrie Sky

MMOACT = Massively Multiplayer Online Action Games
We just call them Action MMOs here at FreeMMORPGs.com. Several games could fit in this category including Lunia, 2 Moons, and more.

MMOSG = Massively Multiplayer Online Social Games
Definitely on the move up as social networking and virtual worlds continue to rise in popularity.

MMOSPT = Massively Multiplayer Online Sports Games
This sub-genre is growing but still lacks a huge mainstream game.