There are two key reasons to do backups: data redundancy in case of local hardware failure and data availability in case of disaster. Appropriate RAID arrays can ensure the availability of your data in case of hardware failure. Careful network backups can help keep your data available in case of natural or man-made disaster. Naturally, this is an administrative function beyond the capabilities of regular users.
High-speed network backups saved the data from a number of financial firms after the tragedies of September 11, 2001. Without those backups, a lot more financial data would have been lost, and I suspect the subsequent economic declines might have been much worse.
Many standard books and documents tell you how to back up your system while it's down and unavailable to users. But Linux is increasingly being used as a server in environments where downtime is considered a sin. When used with removable hard drives, RAID does not require downtime. And the removable hard drives can be sent to safe locations.
Unfortunately, hardware RAID solutions are more expensive compared to software RAID. And they go beyond the packages that are included with most Linux distributions. There are other high-capacity/high-availability commercial solutions, such as Red Hat Cluster Manager and SUSE Heartbeat. But if you're stuck and need to configure a real-time backup using just the software available with a Linux distribution, consider software RAID, as described ...