DRM. and its complete bollocks and doesn't apply natively to MP3 you have to get it applied via the software you download it via onto the device they are playing it through. But ultimately if it can be played through a speaker it can be copied at high quality with no loss.

You would be a complete monkey to go down the DRM route - just sign-up for a service like Bleep.com and go for volume sales of good music not being a twat and trying to lock down using DRM.