I have a Lenovo x200 with an external RW drive. When I put in a DVD, it opens to folders and the media's pieces are there but you cannot click them to get them to start. The gnome player will not play the DVD. I cannot figure it out.
The codecs for watching DVD's are either not open sourced, or are not free, which is why they are not included with Ubuntu. If you wish to play DVD's specifically on your computer, you will need to install them. Installation instructions can be found on the link below.
Of course, there are other codecs that are restricted as well (AAC/m4a), and for that you can follow the instructions in the other answer for those codecs.