It is perceived to have artificial butter, butterscotch, caramel, or toffee aroma and flavors. Sometimes it expresses as a milky slickness on the tongue. It is caused by inadequate boiling or oxygenation of the wort. 3-7 days of Diacetyl rest (slightly raising the wort temperature ~20-25OC towards to end of fermentation) helps in controlling it, as the yeast will naturally metabolize these away. Diacetyl is often confused with kettle caramelized malty notes. A bit of training can help isolate the two very different faults. If not taken care of, this can cause serious hangover or headache in beers.