OGG is an audio format based on the free OGG Vorbis compression method and, although similar to MP3, OGGs will sound better than MP3 files of the same size.
OGG is a container multimedia format and it may include audio, video, subtitles and metadata. OGG is used particularly for streaming and managing high quality media.
The OGG format is widely supported by a lot of multimedia apps, therefore it is understandable why users will convert their files to this format.
Another way to generate custom OGG files is to capture an audio stream on to your Mac and save the resulting track as an OGG file.