In order to play DVDs you must install some additional software. Unfortunately, DVD support cannot be provided by default in Ubuntu due to legal restrictions in some countries.
Read about restricted formats before following the instructions below. There are some legal issues that you should be aware of.
Install the libdvdnav4, libdvdread4, gstreamer0.10-plugins-bad and gstreamer0.10-plugins-ugly packages.
If you would like to play encrypted DVDs, press Applications → Accessories → Terminal and type the following into the screen which appears, followed by the Enter key:
Enter your password if prompted. The libdvdcss2 package will be downloaded and installed from a website.
Insert a DVD into your drive. It should open automatically in the Movie Player.