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In all likelihood you know what a ringtone is and that you purchase and install new ones on your cell phone or smartphone, if not regularly, then occasionally. Do you know what a ringback tone is, though, let alone that you can buy and load new ones to further personalize your mobile phone? Although you're far from alone if you don't, ringbacks are growing in popularity and are become increasingly important to the music industry.

For instance, at CTIA last week, the RCA Music Group announced Alica Keys hit single "No One" recently became the first ever Gold-selling ringback. This means sales of snippets of the song to be used as a ringback exceeded 500,000 copies; the same number you'd have to move to achieve Gold status in the album, single and - yes - ringtone charts.

So what exactly are ringbacks? Here's the short course:

While a ringtone is the sound you hear when someone calls you, a ringback is the sound - usually ringing - you hear when calling someone else. While ringtones, often in the form of a song clip, are among the most popular content purchases for cell phones and smartphones, you can also alter the ringbacks people hear when calling you.

That way when folks call, they'll be treated to something different than the usual ring ring ring ring before you answer or your voice mail picks up. So, for example, if your sweetheart calls, you can set the ringback to allow her to hear your and her favorite song together.

For the music industry, ringtones and ringbacks fall into a larger category called mastertones, combining sales of both types of downloads. Keys "No One" has reached Double Platinum in the mastertone category by surpassing 2 million in sales.

In a report last month, reported on by BetaNews, IDC predicted ringback sales would overtake ringtones by 2010 to become the largest source of mobile entertainment revenue.

IDC projects wireless service revenues to hit an amazing $800 billion by 2011. $40 billion, or 5.1 percent of that, will involve mobile entertainment.

As the article points out, you can choose a different song or clip for every caller and change them often. And, like with ringtones, it's not just about songs, but comedy and celebrity voice content as well, for example.