Wow, people get anal over this kind of crap.
In most games I've played, DPS means any class that deals damage, but I find that misleading. Damage Per Second should mean a class that deals weak damage quickly, like thieves/rogues/archers/ninjas and such.
DD is the same thing, only it's used in fewer games. Yes, I HAVE heard it used in some of the more backwoods games, but it is used.
And, if you want to get technical, there's always spikers, DoTs, AOEs, and DB. DPS is a broad term, DD is specific. Example: Mages often don't deal fast damage, but deal large amounts of damage slowly. Why are they called DPS, then? If you want to go apesh*t about it, then DPS should refer to the overall party makeup and not a particular class.
Anyway...
On topic...
What was the question?
Oh, right...
MMOs where classes need to work together? Rappelz, Requiem, FLYFF (never enough RMs), Rohan, Silkroad (Euro classes), and Perfect World to name a few.
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Main games I've played: KalOnline, Requiem, Silkroad, FLYFF, Rohan, Rappelz, and RF Online
Currently playing: SUN Online/Allods (US)