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 WAV?
WAV is raw PCM audio with a small header - uncompressed, unprocessed and enormous, at roughly ten megabytes a minute for CD-quality stereo. It is the working format of audio editing, sampling and speech tooling, where every stage wants the samples themselves rather than a codec's approximation.