Nutmeg is the dried seed of Myristica fragrans. Mace and nutmeg are one in the same, in that, they grow together and once you crack open the fruit that holds that nutmeg kernel theres a lacy webbing surrounding it. That outer layer of the nutmeg kernel is called the aril and that, is where wonderfully smelling, delicious tasting mace comes from.
Mace is the sweeter flavoring, and subtler smelling sister of nutmeg, although it can be used as a substitute to nutmeg in most recipes.
Ground mace can be used in lightly colored sauces, broths, mashed potatoes, and should be used in the beginning stages of the cooking process to allow for the full flavor to be achieved.