MP3 is an audio file that has been compressed to take up less space on a storage device. It is the most popular way to download audio online. Due to this, it has also been at the middle of several lawsuits for illegal downloads on peer-to-peer networks.
The quality of MP3's are not on par with the original. The bit rate of an MP3 is the amount of audio information reproduced per second. The higher the bit rate, the better the quality of audio.