In the formative years of Unix, there was only one reasonable kind of backup hardware: the tape drive. Tapes were inexpensive compared to fixed storage (hard drives), they were fairly reliable, and they could be taken off-site for additional data security.
However, hard disk technology has outpaced tape drive technology in the past ten years. For PCs, tape drives are expensive compared to hard disks (and the tapes themselves are also expensive). In addition, tape drive capacities are far smaller than the disks now on the market.
There are currently two popular backup alternatives to tape drives:
CD- and DVD-R/RW drives
An extra hard disk (internal or external)
Both of these work, but you need to improvise, because there aren't any tried-and-true standard solutions for making backups to these devices. You still want to use a traditional backup utility (almost certainly tar) to create archives on the medium, but you need to find a way to manage the archives by yourself.
That said, don't count out the tape drive just yet. A significant portion of this chapter deals with tape drives.