MPEG-2 (aka H.222/H.262 as defined by the ITU) is a standard for the generic coding of moving pictures and ISO/IEC 13818 MPEG-2 at the ISO Store. It describes a combination of lossy video compression and lossy audio data compression methods, which permit storage and transmission of movies using currently available storage media and transmission bandwidth.
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.