You may have put a DVD into your computer and assumed that just because you have a DVD that you can simply open it in Windows Media Player, but have found that it'll give you an error or simply won't open. This is because Windows Media Player doesn't include a DVD Decoded within it.
A DVD Decoder simply decodes the data stream read from your DVD and pipes it in a format that can be played through a DVD Player on your computer. It does this in 2 ways:
-- Decodes the CSS Encoding which is applied to all video files on disk.
-- Decodes the MPEG2 Encoding which is applied to video content on disk.
There are many DVD Decoders to choose from online, however not many are free; in fact, you'll find that most are not. This is because of the development and licensing costs paid to the DVD patent holders. As most DVD drives will come with some software to play DVD Videos, this is a definite "free" option.
Microsoft has a list of DVD Decoders which they recommend that you can access here. Once you've installed a DVD Decoder, you'll be able to (properly) watch DVD's within Windows Media Player.
Another option is to use another program, such as VLC Media Player which has a DVD Decoder (as well as many other codec's) programmed into it.