What is FLAC?
Lossless compression for audio: about half the size of a WAV, with every sample bit-for-bit intact. It's what people archive CDs into and what hi-res download stores sell. If a conversion starts from FLAC, it's starting from a perfect copy.
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.