Di - In order to store the amount of information in an entire movie on a DVD, they use compression. A DVD decoder is the software (or hardware in some cases) that decodes this compressed data into normal data so that you can play DVDs (a home theatre DVD does this with hardware, a computer usually does it with software). They typically cost $15.
ivnj - DVD decrypter doesn't look like what I'm looking for. I'm only looking to play DVDs, not copy them. I have interactual player, but I still need a decoder before it is usable.