The Naturalistic Fallacy is an ontological principle which states that 'it is impossible to derive an ought from an is'. In more general terms, it is impossible to logically progress from an observation of the physical world to a normative ethical system or make moral statements.
It was very roughly proposed by Hume, and is also known as 'Hume's Guillotine', but later expanded on in much greater detail by Moore. He observed that all moral statements, most clearly psuedo-syllogisms, progress using flawed implicit logic.
He then went on to argue that there is no way of logically showing the premise that the murder of innocents is 'wrong'- there is no logical argument which can be made to show this is true or false. Moore, however, was not arguing that ethics is fatally flawed, but argued that this was a problem that affected most normative ethical systems and needed to be addressed.