What is M4A?
M4A is AAC audio in an MP4 container - what iTunes sold, what Apple Music and most podcasts ship, and what a voice memo becomes. At the same bitrate it sounds better than MP3, and it is read by rather less hardware.
What is AAC?
AAC is MP3's designated successor and the audio in most video: YouTube, streaming services, broadcast and virtually every MP4. At a given bitrate it beats MP3 audibly, particularly below 128 kbps, and every phone and browser decodes it.