Last week we learned that Adele's 2011 blockbuster 21 was the top-selling album of 2012, with 4.4 million copies sold. It was the second consecutive year that 21 was the top seller -- a first since Soundscan began measuring album sales in 1991.

Soundcheck regular, and Billboard editor Joe Levy explains the success of 21 this way:

"The theory in the music industry is that when you have an album that 14-year-old girls want to buy for their moms, and moms want to buy for their 14-year-old daughters, you’ve got a big hit on your hands because you have a record that appeals to everyone.”

Levy also explains what the other year-end numbers have to say about the state of the music industry and its consumers.