What is a fruiting body? The fruiting body is the mushroom itself.
What is a mycelium? You could compare mycelium to the root system of a tree.
Honest mushroom products are made from mushroom fruiting bodies, which are rich in the compounds of interest present in mushrooms.
Note. Myceliated grain products are also available on the market, they contain mainly starch from grains and only minimal amounts of active compounds. These are rather food products than nutritional supplements, although some of these products are marketed as mushroom supplements, they lack many of the beneficial compounds.