X (or XXX) usually is the rating for an 'adult' film. NC-17 is a higher rating than an R, but usually, the directors like to fight it.

From the devils own website -

"Second, we changed the name of the X category to NC-17:NO ONE 17 AND UNDER ADMITTED. The X rating over the years appeared to have taken on a surly meaning in the minds of many people, a meaning that was never intended when we created the system. Therefore, we chose to reaffirm the original intent of the design we installed on November 1, 1968, in which this "adults only" category explicitly describes a movie that most parents would want to have barred to viewing by their children. That was and is our goal, nothing more, nothing less.

We have now trademarked "NC-17:NO ONE 17 AND UNDER ADMITTED" so that this rating symbol and the legend can be used only by those who submit their films for rating."

This is why the director's cut of Robocop is sometimes referred to as X-rated. Now that the system has been changed, it would be rated NC-17 for violence.

This also explains why the porno's that play on canadian TV at night are rated NC-17 and not X....lol If you wanna know the diff between XX and XXX, look that up yourself