DVD playback is not a feature that is installed by default due to license restrictions. This tutorial will show you how to install DVD playback capability on your Ubuntu linux machine.
Read it here
note: in some areas this is not legal. If that is the case for you you can 1) continue to let some corporation demand payment to be able to playback the DVDs you already paid for, 2) stop watching DVDs altogether or 3) tell them to take a hike and install playback anyway. Viva la revolucion!