The idea is that these are saved in a different format. Commonly used is JPeg but a growing number of photographers now use DNG.
So why not RAW or Tiff?
Well RAW is just that an unedited RAW file not your final image unless you also bundle into your archive the database for Adobe Lightroom or Apple Aperture.
RAW files are a proprietary format that may not be supported in the future.
TIFF is a good solution if you have the harddisk space, JPeg is smaller but lossy
I find the ideal compromise is DNG. Now this is a standard that Adobe control but it is open and the specification is well published so many program’s can read and view DNG’s.
So if you have a spare drive or a internet storage solution, save your favourite photos as Jpegs or in DNG. Just in case.
If it’s an old drive ask if you can store it at your parents or brother or sisters house. If you have more then one drive rotate them round.