Most commercial DVDs are dual layer and contain more than the 4.7GB that you can fit on to a standard recordable DVD. This means you cannot simply copy the DVD. The program suggested by MAJ will decode the DVD and then re-encode it at a lower quality such that it will fit on a recordable DVD. The quality achieved is perfectly acceptable and I would be surprised if you notice the difference.
Have a look at this site for further information click here
click here takes you directly to the guides for DVD back-up.