Backup 2.0.1 adds quality and reliability improvements to scheduled backups and is recommended for all Backup 2.0 users. Please use either Software Update in System Preferences or download Backup 2.0.1 with the button below to bring your system up to date. If you haven’t started using Backup yet, let us take this opportunity to urge you again to do so. Data loss seems distant and remote until it happens to you, and then it feels harsh and painful. Backup 2.0.1 gives you the ability to protect your important data via your iDisk, CDs and DVDs, and external FireWire hard drives (including iPods), network servers, and other mounted volumes. And it’s way too easy to use for you not to take advantage of it.
Backup 2 requires the latest update of Mac OS X version 10.2 or 10.3; Backup 1.1 requires Mac OS X version 10.1.5 or later. Backup 1.1 users must upgrade to Backup 2 to access the following features: backing up to hard drive, backing up to external (OS-supported) external optical drives, disc spanning, and numerous QuickPicks. Non-standard (symlinked) Home folder configurations are not supported.
Before installation, make sure you have the latest Mac OS X update. Choose System Preferences from the Apple menu, then click Software Update and proceed with the update.
Backup requires a .Mac account. Full .Mac members can use all Backup features. Trial accounts can back up only to iDisk.
.Mac members using Mac OS X version 10.1.5 should download and install Backup 1.1.
More info here.