Often when bands release a single, they want to put a little extra on there for the fans. It so happens that back in the days of the 45 rpm record, there was a whole half of the vinyl record left over for the extra music; the side with the main song on it was the "A-side" and the side with the rest on is the B-side.
Even today, now that the 45 rpm single is more-or-less on the way out, the [...] [tvtropes.org]