There are many audio file formats of which the first was MPEG-1 Audio Layer 3 or more commonly referred to as MP3. It is a lossy compression algorithm that removes certain frequencies that humans can't hear. When creating an MP3 file, a bit rate is set that has a big effect on the quality of the sound. Setting a bit rate that is too low can produce a poor sound quality.
The term MP3 has become synonymous with digital music files and is the de-facto standard that everything else is compared to. Interestingly this lossy compression algorithm was invented by a group of European engineers who used a component from an earlier invention as early as 1979.

