I'd appreciate it if someone could please explain what a DAC is. I understand its a digital to analogue converter, but why would ytou get one if a regular receiver has a built in DAC? In addition, are DACs limited to audio use (music/cd) alone or will they work with video (i.e DVD). How do they or can they be integrated into a system which streams media such as the apple tv or xbox 360?
I ask because I'm thinking of stepping up from a big box store receiver and I've seen alot of discussions about DACs and preamps which I don't understand.

Thanks

p.s I did search the forums for DAC and all I came up with were questions about recommendations.