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 AC3?
AC3 (Dolby Audio Coding 3) is a 5.1 format, which means that it provides five full-bandwidth channels, front left, front right, center, surround left, and surround right. A low-frequency effect (LFE) channel is included for the sound needed for special effects. AC3 was the first coding system designed specifically for multichannel digital audio. AC3 is the sound format for digital television ( DTV ), digital versatile discs ( DVD), high definition television ( HDTV ), and digital cable and satellite transmissions. AC3 supports audio sampling rate up to 48khz.