M4A is a file extension for an audio file encoded with advanced audio coding (AAC) which is a lossy compression. M4A was generally intended as the successor to MP3, which had not been originally designed for audio only but was layer III in an MPEG 1 or 2 video files. M4A stands for MPEG 4 Audio.
The M4A file type is primarily associated with 'MPEG-4 Audio Layer'. Files in .M4A format are actually the audio layer of (non-video) MPEG 4 movies. M4A is slated to become the new standard for audio file compression. This format is also known as Apple Lossless, Apple Lossless Encoder, or ALE. It is a new codec designed to provide lossless encoding in less storage space.
What is M4A and MP4?
-- MPEG-4 files with audio and video generally use the standard .mp4 extension.
-- Audio-only MPEG-4 files generally have a .m4a extension. This is especially true of non-protected content.
The existence of two different filename extensions, .MP4 and .M4A, for naming audio-only MP4 files has been a source of confusion among users and multimedia playback software. Some file managers, such as Windows Explorer, look up the media type and associated applications of a file based on its filename extension
In response, Apple Inc. started using and popularizing the .m4a filename extension, which is used for MP4 containers with audio data in the Advanced Audio Coding (AAC) or its own Apple Lossless (ALE, ALAC) formats. Software capable of audio/video playback should recognize files with either .m4a or .mp4 filename extensions, as would be expected, since there are no file format differences between the two.
What is M4A and M4P?
M4A files are similar to .MP4 video files since both file types use the MPEG-4 container format. However, M4A files only contain audio data. The use of the ".m4a" extension arose due to the confusion between MPEG-4 video files and MPEG-4 audio-only files.
The iTunes Store provides songs in the M4A format. These songs are encoded using AAC compression, which significantly reduces the size of each file. Songs that include copyright protection have an .M4P extension.
Here, How to Convert M4A to MP3 Mac and MP4 to MP3 Mac