Data rot refers mainly to problems with the medium on which information is stored. Over time, things like temperature, humidity, exposure to light, being stored not-very-good locations like moldy basements, make this information very difficult to read.
The second aspect of data rot is actually finding the machines to read them. And that is a real problem. If you think of the 8-track tape player, for example, basically the only way you can find 8-track cartridges is in a flea market or a garage sale.
The problem, strangely enough, is not so bad on the older stuff, but quite bad on the more recent stuff. So we can read tapes here at the museum that are 50 years old. ...
Making lots of backups is good advice, and on different formats, different places; consider paper as an archival medium. Some paper we have has lasted thousands of years. If Moses had gotten the Ten Commandments on a floppy disk, it would never have made it to today.
--Dag Spicer, NYT, on the glory of paper