Excluding System Files
You can choose to exclude the Mac OS X system files from your backup. This can save a few gigabytes of space, but it will mean that you can't completely restore your computer from a backup (you would first need to re-install Mac OS X).
To exclude system files (including Unix tools), add the System folder located at the root level of your hard drive to the "Do not back up" list. Time Machine will ask if you want exclude just the System folder itself or all system files.