A few years ago the FDA pushed for the use of a standardized over the counter medication label in order to help consumers better understand the drugs they were using (or at least purchasing over the counter).
Not to be outdone, the EFF recently compiled their own standard...a list of the DRM (digital rights management) and other restrictions that the various online music stores (iTunes, Napster, Microsoft, Real) use. An example of such a restriction (embedded within the music file itself) is the number of different computers you can play the music file on (unlike a CD which you could use anywhere you want).
Maybe next time you purchase music online...you should first read the label.