Photographic prints and slides fade, even when properly packaged and stored in a dark, cool place. Color photographs are especially susceptible! When a color photograph fades, the colors change, and image details disappear. The dyes used to print the photo (or slide) are not stable and slowly lose their color. Different colored dyes lose their color at different rates, causing changes to the color of the image. For example, the yellow dye is notorious for its rapid loss of color, and is critical component of some skin tones. Take the yellow away from a picture of a Swedish sailor, and now you have a red or purple sailor. The film industry made great improvements to dye stability in the 1990's, but the problem was so bad that any improvement was significant. Here are some links with more on the subject:
Collection of photograph permanence articles from Wilhelm Imaging Research.
Popular Photography magzine Article on Photograph fade from 1990.
Digital image files do not fade, but the media they are on (magnetic disk, Optical disk or jump drive) slowly lose their minds. In addition, the digital world changes, making some of the media obsolete. For instance, in the 1990's most data was saved 0n 5.5 inch "Floopy" disks. In the 1980's it was 5 inch floppies. The data on those disks is probably still good, but it would be hard to find a disk drive that read the thing. These Fromats seemed to last only a decade or so! It is important that your digital images are on the media that will be both stable and accessible for a lot longer than a decade.
Here at Digital Photographic Services, we put you images on a special grade of CD or DVD that will not degrade for than 50 year's.
The computer industry has learned from the Floppy disk era, and manufacturers now require "backwards compatibility" from optical disk suppliers. This means that your brand new computer with a latest generation of optical disk (Blue Ray) can also read the decade old disks you have. Additionally, optical disks are here to stay. There is nothing else on the horizon that can beat the combination of manufacturing cost and data density of these things.