F2P : free to play (financed by an ingame Item Shop where you'll pay - often through the nose - for individual items like costumes, power-ups, additional character classes etc.)
P2P : pay to play (monthly subscription fee)
B2P : buy to play (installing the client requires buying the software on DVD or over a download portal like Steam, e.g., just like for a singleplayer/offline game; after that, there won't be further subscription costs. May feature an Item Shop, too, but if it does, the Shop will usually be very noticeably cheaper than in an F2P.)
WoW : World of Warcraft (P2P, and the only MMO for which I don't consider this likely to change that in any foreseeable future)
SWTOR : Star Wars The Old Republic (just launched last month; P2P, not sure if they'll make it without switching to F2P sooner or later)
GW2 : Guild Wars II (the classic B2P MMO)
AION : the actual game name, even if it looks like an acronym 😉
triple-A : the big budget production, high-quality games. In the MMO sector, it basically boils down to WoW, SWTOR, GW / GW2, and Lord Of The Rings Online (the latter has switched to F2P about a year ago too, though), even though there's tons of other games of lesser quality (most of them, unsurprisingly, F2P).