Since the body of the mushroom is usually dispersed over a relatively large area it is rarely noticed. Mushrooms are fungi, and are usually placed in a Kingdom of there own apart from plants and animals. Mushrooms contain no chlorophyll and most are considered saprophytes. That is, they obtain their nutrition from matabolizing non living organic matter. The body of the mushroom stores nutrients and other essential compounds, and when enough material is stored and the conditions are right they start to fruit - produce mushrooms.
Mushrooms develop differently than plants or animals do.
Plants and animals grow through cell division - to get bigger they have to produce more cells. Cell division is relatively slow and requires a lot of energy. The mushroom body also grows by cell division. However, the mushroom fruit does not grow by cell division. Just about as soon as it starts to develop, a mushroom has almost the same number of cells that the mature mushroom will have.