What's the special arcane buff?

I've read numerous times: go arcane if your team needs the buff.

I was discussing with my raid leader tonight whether I should go arcane or fire, and realized: I'm not exactly sure what buff this is!!!

I assumed this buff was arcane brilliance, but switching specs from arcane to fire, I didn't see any change there.

I do know I lose focus magic as arcane - is that it?

This is kinda important for me because I'd like to go fire for certain fights to improve my DPS, so help for this "noob" question is very appreciated. Thanks!
The buff is Arcane Tactics, a raid wide 3% damage increase, ret pallies and, I think, BM hunters have it too, so if one of those are in your group, you won't loose the buff.
What Cynn said, for hunters its called Ferocious Inspiration, not entirely sure about pallies but my mind it throwing out something starting with Sanctified. I wouldn't worry too much about having to go Arcane, if you find that switching specs is a personal dps loss then just remember that its a much larger raid dps gain, not to mention that if you have a reasonable computer it'll let you watch a movie for comfortably while raiding :p
ok, well our palladin is a tank, not ret.

BUT - my raid leader is ok with me going fire for certain fights.