PC and XBox have been mentioned, on PS3, it's the R2 button. It doesn't take a "slot" because it's basically your voice, so if anything, your character would have to glow out of the mouth or something.
Same as Dragon Shouts on PC, activate by pressing 'Z'. Same as dragon shouts, they don't take up a hand slot, but rather you can only have one shout/battlecry 'active', equipped and ready to go.