DRM (Digital Rights Management)
"iTunes controls how a track purchased from its music store is used. FairPlay is the brand name of the technology used by Apple; this ensures that the rights are fair to the artist, the record company and the consumer." Every track in the iTunes music store is encoded with FairPlay. This means that the encoded track can be played on up to five authorised computers simultaneously, and a playlist containing a purchased track can be burned up to seven times. If you manage to hit those limits, you will need to buy the track again.