A-side and B-side originally referred to the two sides of 7 inch vinyl records on which singles were released beginning in the 1950s. A-side being the featured song (the one that the record producer hopes will receive radio airplay and become a "hit"), while the B-side, or flipside, is a secondary song that often does not appear on a band's LP.
See also the b sides, b side, flip sides, and b-sides tags. .