NADH and FADH2 are nucleotide derivatives. They are the major electron carriers in the oxidation of nutrient molecules. Their role is to transfer reduction potential. NAD+ is synthesized from vitamin B3 (niacin). We say that NADH is an 'electron carrier', but what happens when NAD+ is reduced is that it receives the donation of a hydride from the mebolite involved in glycolysis or the citric acid cycle. The type of enzyme involved in the hydride transfer is a dehydrogenase.

An interesting aspect of the hydride (electron) transfer is that the nicotinamide ring in NAD+ is aromatic, while in NADH it is not. The aromatic stability of NAD+ is key to understanding why NADH donates its electron so easily (either in fermenatation in anaerobic respiration or to the electron transport system in aerobic metabolism).