What is MP4?

The container video actually travels in: what phones record, what messaging apps accept, what every player since the iPod era opens. It usually carries H.264 video and AAC audio. Its one structural quirk matters to converters - the index of every frame lives in a table that has to be exactly right, or nothing plays.

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.