What's a b-side?

A long time ago, probably before you were born, a song would be released on a small record called a 45.
The record has 2 sides, the A side & the B side
The song on the A side was the new release and the B side was a tune not as will liked by the producer or artist.

You're 'irrelevant to music'? You mean you're 'ignorant about music'?

Originally, when recordings were on vinyl disks, small disks were called 45's, short for 45 revolutions per minute. They had a single song on each side, with side A being the more popular song and side B being the less popular by the same artist.

Records were the music media from the early recording days all the way through the 70's. The singles had an A-side and the b-side, with the A-side the featured song and the B-side a song giving an idea of what the album or artist usually sounds like.
http://en.wikipedia.org/wiki/A-side_and_B-side

Classical notation