Software used to create copies (
backups) of your data, in order to protect it from
loss,
theft,
fire,
acts of God, etc. etc.
If you are reading this, chances are you have a computer, with some stuff on it you would be sad if you lost. If you haven't already, you should get some backup software and a CD-RW or a cheap tape drive today, and make some backups. Preferably multiple sets, some of which you don't keep in the same place as the computer (i.e. safe deposit box, desk at work, etc). Trust me, you'll start doing this the first time you lose everything; why not start early?
There is an unfortunate variety of backup software, much of which is, for some reason, of really atrocious quality. Your ideal backup software should let you use whatever hardware you have to back things up on, and it should support what are called incremental backups.
What do I use? Retrospect.
Notes about unix:
dump and restore aren't all that great. For instance, their support of incremental backups is... well... atrocious. Also, for linux users, the linux/ext2 port of dump is of especially tenuous quality. Make sure you visit the dump section of sourceforge and pull the very latest build. And cross your fingers.