The M4A file format is simply an MPEG-4 file with only a single track of AAC audio. It’s not “. mp4” simply to make it easy to tell it’s an audio-only file. M4A was made famous by Apple with iTunes, and has become broadly supported in media players and consumer electronics devices.
The main advantage of M4A over MP3 is better compression efficiency. In its typical AAC-LC profile, it provides the same quality in a third less bitrate.
An iTunes .m4p file is a FairPlay DRM-encrypted M4A and as such isn’t interoperable outside of the Apple ecosystem. It’s something only Apple can make, and only Apple products can play it back.
AAC Low Complexity (AAC LC) ...