Originally, Apple copy-protected all of the songs in its iTunes Store by using its proprietary Fairplay DRM protection system. This severely limited the choice of iPod alternative players that could be used to play songs purchased and downloaded from its iTunes music library. Compatibility was always poor because Apple restricted the use of their DRM system to just a handful of non-iPod devices and therefore most hardware manufacturers couldn't integrate support for the .M4P file format. Consumers had therefore few options regarding hardware and had to choose either an iPod, or go for an MP3 player for use with other digital music services
For more information, read the iTunes Store review that gives you a full rundown of what the iTunes Store has to offer.