CD-R - user burnable CDs.
  To provide recording on a PC, the CD-R was created.
    Uses a dye in place of a real pit.

    Uses a two level laser that can either read the pit or burn the dye to 
      create a new "pit".

   Contains a second embedded track that provides some ballpark timing for the 
     CD-Burner.

Because data is actually just under the top side of the disk

  Scratching or wearing the top can be more damaging than damaging the
  clear underside.
     
  Using a paper label protects data.
    Burn CD before applying label.  Additional material can affect 
      rotational speed when recording.

    Improperly applied labels can unbalance disc and/or stress surface. 

    Mark label before placing on disk to prevent physical damage.

    Don't use solvent pens directly on disk and make sure paper label dry
    before applying to disk.

    * Sharpie claims to be alcohol based ink that doesn't seem to attack CDs.

    Some CD makers also market water based markers for marking CDs.  

  Some newer CD-r's claim to have writable surfaces.
    Ball point pen can indent surface.
      Solvent from permanent markers can attack disk.

  At this time, the markers are actually the better choice.

  Light-scribe - newest CD burners - top of CD/DVD use dye similar to data layer.
    May have 2nd laser on top that burns label layer.
    Or require you insert CD upside down to write label.
  
  Lifespan
    Reflective layer often silver which can tarnish over time.

    Dies degrade - some are more sensitive to UV.
      May be hard to find which brand/batch uses which die.

    Layers can separate.

    In theory - accelerated tests imply 50-100 Yrs.

    Reality - new technology.

    Don't expect commercial CDs to last longer that 10-15 Yrs.

    Don't expect CD-rs to last longer that 5-10 Yrs.

  For backup :
    2012
      100 CD-r < $20  (65 GB + time to burn)

      100 DVD-r < $25 (470 GB + time to burn)

      500 GB external hard drive < $60 (2013)
        Drives reasonably cheap, reusable, more trust-able.
        Should be read periodically (twice a year) to refresh.
          Do a search on "is data refreshed when hard drive read"
          Windows 7
            for /r %x in (*) do type "%x" > NUL 
            (keep in mind, this won't be fast)

      500 GB Tape @ $10-$40 - but requires tape drive $200-$1000 and time.
        Tape slowest backup/retrieval and possibly 40% failure after
        2-3 years. Also becoming obsolete.

      256 GB SSD $170 

      16 GB flash $12 (3x cost of cd)
         - fast, expensive, reusable, longevity not truly proven.