Asynchronous vs Synchronous


What's the diff between an asynchronous and synchronous DACS? Appears that asynchronous ones are "better". Can someone explain the difference in really basic terms? Talk down to me, please! I have a Bryston BPA-1/BDA-1 combo. I'm guessing that my Bryston is not asynchronous.
Thanks
rockyboy
Bryston makes an external usb to spdif converter the BUC-1 that can be used with the BDA-1. I own the bdp-1/bda-1 combo too. I haver used the M2tech Hiface in the past with good results.
George
From all the responses, I'm concluding that I don't need to worry about whether my DAC is asynchronous or synchronous, since a PC is not involved in my system. FWIW, I'm linking my BDP & BDA with an AES/EBU cable.