http://www.cuyamaca.edu/bruce.thompson/Fallacies/naturalistic.asp
Also, from the Routledge Dictionary of Philosophy: "Modern discussions start from two famous attacks on these two forms respectively, by Moore and Hume. Moore insisted that 'good' is indefinable, and that the questions what 'good' means and what things are good must be sharply distinguished, e.g. pleasure may be good, and even possibly the only good thing, but 'good' does not mean 'pleasant' or 'producing pleasure', etc. He called the neglect of this distinction the naturalistic fallacy, and said goodness is a nonnatural quality."