Can anyone give some answers on what is disk mirroring in Unix? It may be related to unix online backup.
In a nutshell, disk mirroring is when two disks are written with the same data at the same time. So, when you have critical data (critical SQL databases are a good example) the disks are often mirrored so if one disk crashes, a minimal amount of data is lost. You are correct, disk mirroring is related to system backup and recovery. Usually, backups are performed daily for critical systems. Disk mirroring provides for disk crash protection between backups, in almost real time.
BTW: Does anyone know of any good mirroring software for Linux? I think I'll mirror this site in the future! Many someday, the UNIX For Dummies archives will be 'critical' for users 
The Linux LVM should support this soon, I have heard of some workarounds to get it to work sooner though...
Check out the Software RAID HOWTO also:
http://linuxdocs.org/HOWTOs/Software-RAID-HOWTO.html