If you want to create back-up copies of your commercial DVDs, you need to break the encryption on them.
Search Google for a program called "Mac the Ripper" which will do this and copy the contents of your DVD, decrypted, to your hard disk.
Note that while most places explicitly allow backup copies of purchased digital media, in most places circumventing copy protection is explicitly ILLEGAL, leaving you in the bizarre legal situation that it's legal to own something that's illegal to create (since all commercial DVD's are encrypted).
You can then use the excellent Toast (costs money) to burn this decrypted VIDEO_TS folder to video DVD (and also to shrink it down to fit a single-layer burnable DVD), or you can use the free DVD Imager to create a disk image (.iso), which is then burnable using Disk Utility, which comes with your Mac. DVD Imager will not, however, shrink it down to single-layer size, so you may have to use double-layer disks.
More info can be found in the more appropriate Applications Forum:
http://forums.macnn.com/applications/