Apple Time Machine is a backup program built in to Apple's Mac OS X Leopard operating system. When it's first activated, it creates an exact copy of everything on your computer's hard drive and places is it on a network drive to which your computer is connected via AirPort Wi-Fi. Each subsequent hour, it creates another backup directory on that drive, but copies only the files that have changed since the last backup.

At any time, you can enter a visual interface that shows you how your system looked at each backup point. From there you can restore your entire system or even specific files or folders to a prior state. Since storage space is limited, Time Machine automatically removes the oldest backups as needed.

Time Machine is integrated into a number of other Apple applications, such as iWork, so you can restore files from within those programs. Unfortunately, it does not support backing up to an external hard drive via USB or any other interface besides AirPort Wi-Fi.