CSS stands for Content Scrambling System, which is an encryption system used to protect most commercial DVDs from begin played or copied without authorization.  This scheme effectively locks the DVD and prevents computer software from playing or copying the movie without a key.

Fortunately for consumers, many free decrypting utilities now exist which unlock the DVD and allow software players and copy software to read the DVD. Some of these utilities work on-the-fly (silently in the background) as the DVD copy software copies the movie. One of the more well known free utilities is called DVD43. Once it's installed, all DVDs movies are effectively unlocked. 

Some DVD copy applications incorporate their own decrypting capability, either embedded or via a plug-in architecture.  Although this might seem like a good idea, it leaves the manufacture vulnerable to lawsuits due to the illegal nature of decrypters in many countries. We have already witnessed one company loose a court battle in the USA (321 Studios Inc.) for integrating a decrypter into their product.  Sadly, over a million users of their software are now left with a product which has no support, no updates and no future - the company is now bankrupt.  We feel the best and safest solution for consumers is to use a free third party decrypter like DVD43, which doesn't saddle the DVD copy application with a legal liability.