Files with the M4P extension have limited compatibility and can only be played using iTunes or an iPod. They cannot be converted to play on other MP3 players or with other programs.
M4P files can be copied and moved to different devices, but the DRM that came with the original remains and it will restrict the number of devices that can play the file, even if it was bought legally.
"Cracking" the DRM from M4P files is illegal under the Digital Millennium Copyright Act. However, it is legal to burn DRM-protected files to CD for personal use. And it is legal to rip that burned CD back to your computer, removing the DRM in the process. Audio quality will degrade slightly through this process, however.
In 2007 Apple began selling non-encrypted movies and songs using the M4A file extension. Since April 2009, all songs sold by iTunes have been available as M4A files.
Since the introduction of Apple's DRM-free store, iTunes offers a service that allows users to upgrade their encrypted M4P files to DRM-free M4P files at a cost of 30 cents a song.