As computer users, we are accustomed to thinking in terms of information stored in files. However, not everything is formatted as a computer file. An example that often causes confusion is the music CD. The way the music is stored predates the common use of file formats. The music is stored as raw digital data and not in a computer file. If you view a music CD in your PC you will see CDA files but these file are generated by Windows software and do not actually exist on the CD. To enable copying music from a CD to your PC, it has to be converted to an MP3 or other audio file format.
An excellent explanation of the music CD format (and copy protected DVDs as well) is at this article
Has anyone noticed that the computer revolution has turned its focus from improving productivity to wasting time on trivia?
—John C. Dvorak at PCMag
Source: Daily Infographic