To duplicate an existing install CD, just use Disk Utility to make an image and burn the image. (In Disk Utility's outline view on the left, be sure to select the topmost item for the CD when converting to a disk image, not the "Mac OS X Install" item.)
To make your own bootable CD is a bit more difficult, but there's third-party software to help with this. Look for BootCD on Macupdate or versiontracker.