FLAC stands for Free Lossless Audio Codec, an audio format similar to MP3, but lossless, meaning that audio is compressed in FLAC without any loss in quality. This is similar to how Zip works, except with FLAC you will get much better compression because it is designed specifically for audio, and you can play back compressed FLAC files in your favorite player (or your car or home stereo) just like you would a MP3 file.

FLAC stands out as the fastest and most widely supported lossless audio codec, and the only one that at once is non-proprietary, is unencumbered by patents, has an open-source reference implementation, has a well documented format and API and has several other independent implementations.

xiph.org/flac
xiph.org/flac/comparison
xiph.org/flac/documentation
xiph.org/flac/download
xiph.org/flac/faq
sourceforge.net/projects/flac
github.com/xiph/flac
en.wikipedia.org/wiki/FLAC

Additional resources:

FLAC Test Bench:

Encoders:

FLACCL ~ cue.tools/wiki/FLACCL