MP3 is short for Mpeg3 (like mpg movies) but rather than store image and sound data, it just stores sound data. the format is popular because the files are compressed, meaning that you can effectively fit circa 10 CDs worth normal audio one 1 CD in MP3 format. However, bitrates rary and tehrefore so does quality. the lower the bitrate, the lower the quality but smaller file size. The MP3 player itself merely 'unzips' or 'uncompacts' the files as it plays. this is why 'normal' CD audio playes cannot play MP3 music since they cannot translate the info.