There is an irony about digital photos: they can last forever without degrading (even get better through improved imaging software) and yet they are very volatile, many bits on storage media that can easily get lost or damaged.
A backup is a copy of data that is sufficiently independent of the original so that destructive events can’t affect both at the same time. A backup doesn’t prevent destruction of data; it only allows you to recover the data once the destruction has occurred.
A simple example of backup is copying files from a laptop to a CD (the act of copying), and putting the CD in your desk at home (independence of location). If the laptop is stolen (the destructive event), you still have the data that was copied to it.
To design and implement a backup plan, one has to consider the possible threats to data (e.g., theft, electrical surge, fire); the various ways to copy data (e.g., using the Mac Finder or Windows Explorer, using a backup utility, using CD/DVD-burning software); and ways of achieving independence (e.g., online storage or placing media in a safe-deposit box).
Unfortunately, most articles about backup focus on the copying and ignore data threats and independence. But without evaluating all the threats, there’s no way to be sure that the backup will allow you to recover from them, and insufficient independence means that both the data and the backup can be destroyed by the same event. An obvious example is a fire that destroys everything in an office.
Additionally, if backup isn’t convenient—automatic, ideally—it may not get done often enough to be effective. (It’s common after a data loss for someone to regret that their most recent backup is months old.) The restore has to be convenient, too, or else the damage will be compounded— an electrical surge is bad enough, but if your data is unavailable for a week while you restore it from an online service, you still lose a week of productivity.
Since backup is potentially expensive and time- consuming, you also have to consider the importance of your data. Backing up irreplaceable photographs is more important than backing up application preferences.