The M4R is an iPhone ringtone file that is essentially a renamed AAC (m4a) file. Not all iTunes Music Store songs can be used to create M4R ringtones. In order to create an M4R ring tone from an iTunes song, the song must allow for M4R ringtone creation. An M4R ring tone is automatically transferred to a user's iPhone when the iPhone is synced with the user's computer.
M4R is essentially a renamed AAC (M4A) file - it is similar to MP3 but compresses the digital audio files further. It is part of the MPEG-4 standard and is specified in the ISO/IEC standard 14496-3. It is a lossy algorithm much like AAC and MP3. M4R supports auditory masking - essentially discarding data with minimal loss of quality.