If they are DVDs that you have created and burnt using iDVD or a single-sided DVD without encryption, you can simply copy it using 'Disk Utility' (located in 'Applications' --> 'Utilities').

If not, there is a program called 'Mac the Ripper' on Version Tracker that will let you back-up DVDs, however, I have never used it so I can't give you instructions on how it works.

Good luck,
Sphynx :-)