ogg is the file extension for Ogg Vorbis files. This is an open source patent-free audio compression format that can be used instead of proprietary digital audio encoding formats, such as MP3.
OGG is a bitstream format for saving sounds or music using very little hard disk space. As expected, the last three characters of the file name are ".ogg".
The Ogg Vorbis format is an advanced binary audio format similar to (but more capable than) the mp3 format. It is patent and royalty free, and is supported on most platforms.