M4P, iTunes Music Store Audio File (.m4p)
The M4P Audio file format
The M4P file format is an extension of the MPEG-4 Part 14 or MP4 format of media storage. The best feature of M4P files is that it is a protected file and incorporates the DRM system. Thus, to play the file you need to perform a series of steps to authorize your computer using Apple iTunes.
Technical details of M4P files
Primarily associated with the Protected AAC file produced by Apple Inc., the M4P file uses Apple's "FairPlay" system DRM. This restricts copying the data and requires the authorization of a system via the Apple iTunes software.