Molds, mushrooms, mildews, and yeasts are all classified as fungi, a kingdom of organisms distinct from plants and animals. Fungi differ from plants and animals in several respects. Unlike animals, fungi have cell walls. However, unlike plants, which also have cell walls, fungal cell walls are made mostly of chitin and glucan. Fungi cannot produce their own nutrients as plants do through photosynthesis. Fungi secrete enzymes that digest the material in which the fungi are embedded and absorb the released nutrients. Multicellular fungi do not differentiate into different organs or functional components the way plants and animals do.

Approximately 100,000 species of fungi exists; fewer than 500 fungal species have been described as human pathogens that can cause infections. Visible growth of multicellular fungi consisting of branching filamentous structures (mycelia) are known popularly as molds.

Molds are ubiquitous in nature and grow almost anywhere indoors or outdoors. The overall diversity of fungi is considerable. For example, the genus Aspergillus has at least 185 known species. Molds spread and reproduce by making spores, which are small and lightweight, able to travel through air, capable of resisting dry, adverse environmental conditions, and capable of surviving a long time. The filamentous parts of mold (hyphae) form a network called mycelium, which is observed when a mold is growing on a nutrient source. Although these mycelia are usually firmly attached to whatever the mold is growing on, they can break off, and persons can be exposed to fungal fragments. Some micro-organisms, including molds, also produce characteristic volatile organic compounds (VOCs) or microbial VOCs (mVOCs). Molds also contain substances known as beta glucans; mVOCs and beta glucans might be useful as markers of exposure to molds.

Some molds are capable of producing toxins (sometimes called mycotoxins) under specific environmental conditions, such as competition from other organisms, these types of molds (often classified as black toxic mold) are toxic and should be inspected by a mold remediation professional, after a mold test is conducted a mold removal might be necessary. Molds capable of producing toxins are popularly known as toxigenic molds; however, use of this term is discouraged because even molds known to produce toxins can grow without producing them. Many fungi are capable of toxin production, and different fungi can produce the same toxin. Black toxic mold is an example of this type of mold