First of all, MP3 is a lossy compressed audio format, and everything else is video. ASF is a derivative of WMV in a sense. WMV is characterized by small station space and long video, which is a better format. RM is the format of the previous 56K network speed era, which is unclear but occupies a small space. RMVB is the evolution of RM, and the station space is general but clear! AVI takes up less space and is more suitable for MP4 players supporting this format. MPG should be the original form of MP4 format.