What is the difference between “backup software” such as SuperDuper! ($27.95) and Carbon Copy Cloner (donation ware); and Mac OS X 10.5’s Time Machine?
Backup Software will provide failover support for the all-too-common case where things fail in a pretty catastrophic way, such as when a drive fails, or your system becomes un-bootable. The software accomplishes this by creating a fully bootable copy of your source drive, current to the most recent time you backed up your machine using the software.
Recovery is near immediate, even if the original drive is completely unusable, because you can start up from your backup and continue working.
You can take your backup to a totally different Macintosh, start up from it, and work while your failed Macintosh is in the shop… then, when it comes back all fresh and shiny, restore things and keep working.
Time Machine lets you step backward in time in order to access a file that was subsequently changed or deleted. With this purpose in mind Time Machine performs an hourly backup, saving those files that have undergone change within the preceding hour. Time Machine preserves every file backed up in the last twenty-four hours, then one instance of a backed up file for each day in the most recent week, then one instance of a backed up file for each week.
When the Time Machine volume becomes full, the oldest files will be pruned to make space for new files. What is great about Time Machine is that it gives you a significantly increased likelihood of being able to access information that has, either intentionally or unintentionally, been erased from your internal drive. You can not reboot your system from Time Machine.