Time Machine is the built-in backup tool in MacOS X 10.5. It takes a snapshot of files that have changed (by default it does this once an hour) and copies them to a sparse image on a USB, FireWire, or network volume (this typically requires an Apple Time Capsule drive or a MacOS X 10.5 machine sharing a drive - with a bit of hacking about it can be made to work with cheaper NAS configurations).
