1. Use Disk Utility (located in the Utilities folder; icon is hard disk with stethoscope). Load the disc, and click on the DVD icon in the left panel. Click NEW IMAGE in the icon bar and save your disc image.
2. Use an app such as RipIt (not free) or MacTheRipper (free).
3. If you ever lose or damage your DVD, just drag the image of the disc into the panel in the window, select it and click BURN.
I think the law allows you to have just one backup of your disc, but don’t distribute it; it’s for your use only. Tell me in the comments if I’m right.