Developing and certifying a DVD decoder requires that additional licensing and royalty fees be paid to the various patentholders of DVD technology. This is why DVD decoders are not available as free software. Usually when you buy a DVD player for a computer it comes with DVD decoder software.
So, you might have to pay for a dvd decoder.
There isn't a free standalone dvd encoder, but if you just want a free program to play dvds on your computer, use VLC Media Player.
It doesn't need a decoder to play dvds, so it's the best solution if you want to play them without decoder software.