MP3's is a lossy file format. That means, that in order to make a file smaller, it gets rid of information it thinks isn't necessary.
FLAC is lossless meaning that no information is lost when the file is made smaller (ie is compressed).
FLAC files are larger than MP3, but have better quality.
Queenzone bittorrents are all in FLAC (or other lossless formats) to ensure the best possible quality of the shared material.