WAV is uncompressed, lossless audio. Full quality, large file size. This is the standard for professional production — recording, mixing, mastering, and stem delivery should always use WAV (or AIFF).
MP3 is compressed, lossy audio. Smaller files, but quality is permanently reduced. Fine for casual listening and previews, never for production work.
FLAC is compressed, lossless audio. Same quality as WAV in a smaller file. Great for archiving and high-quality distribution, but not all DAWs support it natively.
When buying stems on SpatialGig, you receive WAV files — the professional standard that preserves full audio quality.