A decoction is made by boiling herbs...usually reserved for twigs, barks, seeds and herbs that are more resilient. An infusion is made by steeping or soaking herbs in a liquid...often that has been boiled, but the herbs themselves are not boiled in it (just like making tea). An infusion is more appropriate for dried or fresh herbal leafs or flowers because boiling removes some of the beneficial properties. Watercress or parsley should be made by infusion rather than decoction.