The fruiting body
A mushroom is the fruiting body of the fungus. It the part of the fungus we see above ground and sometimes eat. Most of part the fungus is living underground and is called mycelium.
The mushroom has a cap that contains the spores. The spores are in the gills underneath the mushroom cap. The cap rests on the stem that grows out of the ground.
When the mushroom is small they are called pin. The pin can grow very fast in to a full size mushroom. If you have your own mushroom grow kit you can actually see the pins grow. If the pin doesn't grow it gets the name abort.
Some mushrooms can be eaten others are poisonous, or psychoactive. The psychoactive mushroom are know under a wide variety of names. Psilocybin Cubensis is the most commonly used and know under the name of magic mushrooms.