What is an M4A file?
M4A is AAC audio stored in an MP4 container, with the extension Apple chose to mark audio-only files. Voice Memos, iTunes purchases, Apple Music downloads, and many recording apps produce M4A. AAC is a newer and slightly more efficient codec than MP3, so an M4A at 128 kbps sounds about as good as an MP3 at 160 kbps.
Where M4A fails
Most modern devices play it, but older MP3 players, some car stereos, certain editing programs, and plenty of upload forms only accept MP3 or WAV. Converting M4A to MP3 takes seconds. For editing, M4A to WAV produces uncompressed audio that any tool loads.
M4A, M4B, M4P, and M4R
They are all the same container with different roles: M4B for audiobooks with chapters, M4P for DRM-protected iTunes purchases from before 2009, which cannot be converted, and M4R for iPhone ringtones, which is literally an M4A with a different extension.
Frequently asked questions
How do I make an iPhone ringtone?
Convert the audio to M4A, rename the extension to .m4r, keep it under 40 seconds, and add it through iTunes or Finder.
Does M4A to MP3 lose quality?
A little, since both are lossy. Use 192 kbps or higher and it will not be audible.