The primary differences are size, quality and ease-of-use. MP3s are the most standard form for audio files and the size of their files dictates the quality. A 128 kbps file is smaller in size than a 256 kbps one, and lower in quality due to compression of the file. FLAC is a lossless format, meaning it will sound as good as any CD. FLAC files do require a plug-in for them to play on your computer and because of that, aren’t as easy to use as an MP3 file.