Ogg is the umbrella for a group of several related multimedia and signal processing projects that are open source and royalty free. Development of these projects is controlled by Xiph.org.
First and best-known project of these is called Ogg Vorbis, a royalty-free audio compression technology.
Ogg Vorbis is an "open-source" digital audio compression format. like MP3, It is a "lossy" compression system, removing frequencies deemed inaudible. Both formats offer variable-bitrate encoding options, for better efficiency. But the algorithms Ogg Vorbis uses to decide which information to discard differ from those used by MP3. Proponents claim that the Ogg Vorbis format outperforms MP3, producing files that are significantly smaller than MP3s of similar sound quality (or files that sound better than similarly sized MP3s).