Fusel alcohols or fuselol, also sometimes called fusel oils in Europe, are a mixture of several alcohols (chiefly amyl alcohol) produced as a by-product of alcoholic fermentation. The word fusel is German for "bad liquor".
Fusel alcohols refer to alcohols that contain more than two carbons. Ethanol (the good stuff) is a two carbon atom molecule. They happen naturally during all fermentations, with higher concentrations occurring when the yeast are under stress (including high temps).