Is a new media container format from Google. Based on video codecs line VP8(9). Its main task is a replacing the old standard MPEG-4. The format is supported by all browsers and is integrated in Adobe Flash technology. For audio codec Vorbis is used; for subtitles and metadata is a the Matroska container.
this format is free, which apparently is one of the reasons for its popularity. The used codecs supports variable bit rates (as a default), and .ogg files usually play back easily on all platforms. All things being equal, the sound quality exceeds that of mp3 somewhat.