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.
What is AAC?
AAC is MP3's designated successor and the audio in most video: YouTube, streaming services, broadcast and virtually every MP4. At a given bitrate it beats MP3 audibly, particularly below 128 kbps, and every phone and browser decodes it.