"audio CD" blanks are basically regular CDRs with a special tax added to the price, supposedly to pay back the music industry for the fact that people use them to copy audio, rather than buying another copy.
However they also contain a bit of information in the ATIP (Absolute Time In Pre-groove) -- which also stores data for a burner telling it what type of dye is used, how much data the disc can hold, et cetera -- which marks them as audio CDRs. A standalone CDR burner will only burn to that type of disc (so that it guarantees that anybody that's using a standalone burner, which is only used to copy CDs, is paying the tax on audio discs).