Reinstalling your Mac's included software
Every new Mac comes with certain programs installed, most notably the iLife media suite. Should you ever need to reinstall these programs, you will need your restore disks. The restore disks should be in the "Designed by Apple in California" package that came in your Mac's box. You will find a pair of disks in the envelope labeled "Everything Else". Insert the first disk into your Mac's CD or DVD drive, and after opening the disc in the Finder, locate a package that says something along the lines of "Install Bundled Software" and open it. DO NOT click on any package labeled "Install Mac OS X", etc.
An installer window should open, and you can go in there and select which programs you would like to put back on your Mac. If needed, your Mac will prompt you for the second restore disk during the installation. Be sure to swap them out when the time comes.