FLAC is a codec for compressing audio data, originally written by Josh Coulson. As the name implies, FLAC compresses data while keeping them identical to the original - this is the main objective of lossless compression algorithms. Digital audio recording (such as CD-Audio track), compressed in FLAC format can be decompressed into an absolutely identical copy of the audio data. The compression ratio for FLAC, usually ranging from 50 to 60% of original size.
FLAC is suitable for daily listening to recordings as well as for archiving because it supports audio tags, album art and embedded CUE sheets. Free and open format make it a well-supported by different software. Support for playback on various devices is rather limited at the moment, but, nevertheless, the format is supported by a large number of hardware devices, compared with a competing format WavPack.
Here you can find some tutorials how to convert FLAC files: