The USB port is used to connect the mixer to a computer for recording or playing back digital audio to/from your computer. The USB port sends hte mixer's main/tape stereo out to the computer. The USB port receives digital audio from the computer; it can then be assigned throught the "tabe/usb to mix" switch to the main left/right output. The USB input level is controlled by the computer volume control.
It doesn't say there's a sound card in it. I've not tried mine to see what it does. I have a Numark mixer/soundcard and a backup behringer sound card.
My son is into music (guitar, bass, drums and now learning keyboard). Figure he'll put the usb to use for recording at some point.
Looking at getting a powered mixer..torn between a mackie (it has DI built in) or a yamaha which I think I like better (and it's cheaper). Will work for me and for his band use should he do more than play in the garage with a friend now and then.