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M4A and M4P are the two main file extensions  for native Apple  audio formats used in iTunes  and iPods . All Apple formats are part of the MPEG-4  multimedia standard which is based on Apple QuickTime . M4A are typically compressed, lossy files using AAC  encoding, and of higher quality than MP3. M4P files are the same but are files specifically purchased through iTunes Store which employ DRM  technology to restrict usage or copying. The M4A extension can also be used to denote the compressed, lossless Apple Lossless  format. This is not a variant of AAC but is encoded in a similar way to FLAC.
You may also come across files with MP4 extension. MP4 is the container format specified in the MPEG-4 standard and can contain either lossy or lossless audio, or video. MP4 is the only official MPEG-4 extension, but most MPEG-4 audio files in circulation have M4A or M4P extension due to their usage in iTunes.

Related articles:

How to import files from iTunes

Exporting your Audacity Project into iTunes and iPod