What is M4A?

AAC audio in an MP4 container, which is what iTunes ripped to and what most podcast apps hand you. It sounds better than MP3 at the same bitrate. It's also the file that gets refused by car stereos and upload forms often enough to keep this converter busy.

What is WAV?

Uncompressed audio, roughly 10MB a minute, and the format every piece of audio software will accept without complaint. Nothing is compressed and nothing is lost, which makes it the right thing to edit and the wrong thing to email.