Molds are organisms that are found indoors and outdoors. They are part of the natural environment and play an important role in our ecological system by breaking down and digesting organic material. Molds are neither plants nor animals. They are part of the kingdom Fungi.
Yeast, mold, mildew and mushrooms are common forms of fungi. Mold is essentially a description of fungi that grows on surfaces, such as the black substance on a moldy shower wall. Mold and mildew often refer to the same type of fungi. All mold is fungi, but not all fungi is mold.
No one knows how many species of fungi exist, but estimates range from the tens of thousands to perhaps 300,000 or more. Some of the more common indoor molds are Penicillium, Aspergillus, Cladosporium and Alternaria.