M4A audio files are very similar to MP4 files, but they do not have a video track, although both of them are encoded using the Apple Lossless Encoder. M4A is actually an audio-only MP4 format version.
Based on the lossless compression which preserves the sound quality regardless of the compression degree, M4a files are used for sharing files over the web, but are not common for playback, therefore they will usually be converted to other formats.
Originally developed by Apple, M4A files are now supported by several 3rd party apps that can be used to quickly convert them to more common file formats.
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