There are two fundamentally different types of statement: statements of fact which describe the way that the world is, and statements of value which describe the way that the world ought to be. The naturalistic fallacy is the alleged fallacy of inferring a statement of the latter kind from a statement of the former kind.
Arguments cannot introduce completely new terms in their conclusions. The argument,
1. All men are mortal,
2. Socrates is a man, therefore
3. Socrates is a philosopher
is clearly invalid; the conclusion obviously doesn’t follow from the premises. This is because the conclusion contains an idea—that of being a philosopher—that isn’t contained in the premises; the premises say nothing about being a philosopher, and so cannot establish a conclusion about being a philosopher.
Arguments that commit the naturalistic fallacy might be flawed in the same way. An argument whose premises merely describe the way that the world is, but whose conclusion describes the way that the world ought to be, arguably also introduce a new term in the conclusion. This is known as a naturalistic fallacy; it remains to be seen whether it will be proven true.
“Democracy is beautiful in theory; in practice it is a fallacy.” – Benito Mussolini