Music encoded as 256kbps AAC files first came to the iTunes Store in 2007 with the launch of Apple’s iTunes Plus. That marked the debut of DRM-free music tracks encoded at a higher quality bitrate that Apple claims is virtually indistinguishable from the original recordings.
As of 2007 the audio files sold in the iTunes store have been encoded using the Advanced Audio Coding (AAC) codec and distributed with .m4a extensions from the iTunes store. The 256 kbps setting is an average bit rate encoding scheme, not a fixed bit rate encoding scheme. The actual sample rate is varied dynamically based on the content and time.