According to HowStuffWorks, a CD-R, or Compact Disc-Recordable, cannot be erased once data is burned on it. The heat used to write data to the disc changes the chemical dyes inside the CD. Because of the chemicals used, this process is permanent and is not reversible.
To have a CD that can be erased and reused, one must start with a CD-RW (Compact Disc-ReWritable). Not all CD burners are capable of writing these rewritable discs, so it is important to confirm that fact before attempting to burn a CD-RW. One should also be aware that CD-RWs may not be compatible with all CD and DVD players.