An audio CD is your typical music CD that you can play in a regular CD player. An MP3 CD is something different, a specially formatted data disc that holds holds MP3 tracks burned from a computer. Both formats have their unique benefits and weaknesses.
The maximum length of an audio CD is 79 minutes. An MP3 CD, on the other hand, can be much longer. Unlike CD audio, MP3s are compressed, and therefore take up much less space. The length of an MP3 CD can vary depending on the length of the songs and their compression rates, but most MP3 CDs can hold well over 100 songs.
The compression of an MP3 CD comes at a price. Most MP3 CDs have inferior audio quality compared to traditional audio CDs. This can be countered by encoding MP3s at a higher bitrate. However, MP3s with a higher bitrate take up more space, limiting the number of songs that can be burned onto a CD.
Nearly every single CD player and CD drive currently in use support CD-R and CD-RW discs that are burned into audio CDs. Many new standalone and car stereo CD players support MP3 CDs, but most older players do not.