A CD stores songs as digital information. The average size of the data for one track on a CD is 32 Mbytes. This is a lot of space for one song, using a standard internet connection it would take over 2 hours to download one song. The MP3 format is a compression system for music. To cut a long story short the MP3 system cuts the size of a track from 32 megabyte around 3 megabytes or so. This lets you download a song in minutes rather than hours, and it lets you store hundreds of songs on your computer's hard disk without taking up that much space. An MP3 is nothing magical. It is simply a file format that compresses a song into a smaller size so it is easier to move around on the Internet and store.