FLAC stands for Free Lossless Audio Codec. It is open source and provides lossless audio compression. This means that even though the audio is compressed, it will still maintain its exact original quality. However, since it is lossless, music stored in this format would definitely take up more space than say MP3 or Musepack audio or any other lossy compression.
Foobar 2000 is an excellent audio player that can play back the FLAC audio format. If you wish to burn music from the FLAC audio format to an audio CD, you can get a plugin for Nero Burning Rom here.
