What is FLAC?
FLAC is lossless: it compresses audio to roughly half its size and gives back the original samples exactly, bit for bit. It is what people archive CD collections in and what high-resolution download stores sell. Sound quality is identical to the source; file size is not.
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.