Ringtones:

Ringtones is very personal thing.  Basically you can choose any piece of audio as a ringtones.  People choose all sorts of ringtones from popular songs, old songs, latest songs, funny sounds, prayers, natural sounds, smooth music etc.

However this particular item is most popular among teenagers and there is a huge market out there for ringtones.

Teens typically keeps on changing their ringtones.  They typically go for hot new popular songs.  Recording companies realized this phenomenon and started licensing the songs for ringtones.  It is an added source for revenue for them.  

Cellular companies also make big bucks by allowing to download ringtones at certain price.

There are number of websites on internet where you can download ringtones.

If you are short on cash, you can actually download a software most of the time free on internet which converts your favorite songs into ringtones.

Ringtones have become multimillion dollar niche market in today's multibillion dollar cellular phone industry.