In order to play DVDs, you need to have the right codecs installed. A codec is a piece of software that allows applications to read a video or audio format. If your movie player software doesn’t find the right codecs, it may offer to install them for you. If not, you’ll have to install the codecs manually — ask for help on how to do this, for example on your Linux distribution’s support forums.

DVDs are also copy-protected using a system called CSS. This prevents you from copying DVDs, but it also prevents you from playing them unless you have extra software to handle the copy protection. This software is available from a number of Linux distributions, but cannot be legally used in all countries. You can buy a commercial DVD decoder that can handle copy protection from Fluendo. It works with Linux and should be legal to use in all countries.

DVDs have a region code, which tells you in which region of the world they are allowed to be played. If the region of your computer’s DVD player does not match the region of the DVD you are trying to play, you won’t be able to play the DVD. For example, if you have a Region 1 DVD player, you will only be allowed to play DVDs from North America.

It is often possible to change the region used by your DVD player, but it can only be done a few times before it locks into one region permanently. To change the DVD region of your computer’s DVD player, use regionset.

You can find more information about DVD region codes on Wikipedia.