A CD or Compact Disc is a circular disk with a silver look to it.
Audio CDs contain audio data, and can be read by a CD player. Any CD marked with the CDDA mark can be played in any player also marked with a CDDA mark.
Audio CDs can be played with VLC if you have a CD drive on your PC. After inserting the CD, run VLC and select Open Disc from the File menu. Then click on the Audio CD option and press OK. If you prefer, you can use the command prompt to run an audio CD:
vlc cdda://D: vlc cdda:///dev/cdrom
Where D: (windows) or /dev/cdrom (linux) is the location of your CD drive.
Data CDs contain programs or files which can be read by your PC - you can only use these with your PC.
CDs can also contain other data and program code. When you insert a CD in some versions of Windows, programs on the CD may run without asking you first - you may wish to turn off autorun or hold the shift key when inserting a CD to stop this happening.
Mixed CDs contain both audio and data, for example a CD may come with a music video as a "bonus feature". The data part (such as the music video) can only be used on your PC, but the audio is able to be played on your PC or CD player.
Some mixed CDs come with programs which will install copy protection on your computer. They need ask your premission to do this. Copy protection may have a detrimental impact on your computer.