What is OGG?
Ogg Vorbis is a high-quality lossy audio compression format. If you need CD-quality sound, neither OGG Vorbis nor MPEG-1 audio layer 3 (nor any other lossy audio codec) can provide exact reproduction; instead, consider using a lossless audio compression scheme like FLAC or APE. Ogg Vorbis is designed for the compression of music and general purpose audio. Special purpose codecs can achieve much greater compression of speech than OGG Vorbis.
Ogg Vorbis will likely be considered one of the most exciting developments in music history. As a creator of music, nothing is more important than getting your music heard. But, how can a creator of music be sure that the music he produces today isn't restricted in the future? Ogg Vorbis is truly open source and royalty free. A musician can create Ogg files and never have to worry about the music being restricted.
If you like Ogg Vorbis, you'd better encode from original, lossless audio sources (like CDs) so that you can get better sound quality. If you have ogg files already and want to listen to ogg songs on portable ipod player, mobile phone, itunes or windows media player, you will need to change ogg files to wma, aac, m4a, mpeg-1 audio layer 3 formats.

What is FLAC?
FLAC stands for Free Lossless Audio Codec, a leading compression technique that preserves original audio quality while reducing file size. The FLAC format reduces the original file size by roughly 30-60% with no loss of quality, hence FLAC is a lossless format.