What is AIFF?
AIFF is Apple's uncompressed audio format - WAV's counterpart from the Mac side of the world, and still what Logic and older Pro Tools sessions use. It stores its sample rate as an 80-bit extended float, a piece of 1980s exotica most converters get wrong.
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.