The M4R file type is primarily associated with 'iPhone' by Apple Inc.. This is basically a renamed AAC (M4A) file.
The AAC file type is primarily associated with 'MPEG-2 Advanced Audio Coding File'. Advanced Audio Coding (AAC) is a standardized, lossy compression and encoding scheme for digital audio. AAC is promoted as the successor to the MP3 format by MP3’s creator, Fraunhofer IIS. AAC’s most noted use is as the default audio format of Apple's iPhone, iPod, iTunes, and the format used for all iTunes Store audio (with extensions for proprietary Digital Rights Management (DRM) where used).