Compact discs are made out of polycarbonate plastic, aluminum and acrylic. Information is stored in microscopic bumps arranged in a single track of data that spirals around the bottom of the disc.
CDA (CD-DA) is a digital storage format designed for CD players. This format can store about 74 minutes of audio at high quality, but it requires large amounts of storage space.
MP3s are compressed files. A CD quality song can be compressed so that one can store many more MP3s in the same space as one could store CDA quality files. This compression is done by eliminating parts of the audio that will not be noticed upon listening to the song.
MP3s can be stored in various bitrates. The lower end is around 128 kbps (kilobits per second) and the higher end is 320 kbps. The higher the bitrate, the more space it will take to store the file and the better sound quality it will have.
At 128 kbps, one megabyte of storage will play about one minute of audio. This translates into around 700 minutes (12 hours) of audio for a 700 megabyte disc. At 320 kbps, this translates into 280 minutes (four and a half hours) of audio. Depending on the bitrate, you can store nearly four times the amount of music using the compressed MP3 format than you can with the standard CDA format, even while using the higher 320 kbps MP3s. With the lower-quality 128 kbps, you can store nearly 9.5 times the amount of music when compared to a CDA format disc.