Fungi can exist as unicellular microscopic organisms called yeasts, as multicellular microscopic molds with hyphae, or as macroscopic mushrooms with a visible sexual organ, the fruiting body.
Some specific fungi can also be dimorphic, present as both molds and yeasts, switching between the two forms according to the environmental conditions such as temperature or CO2 concentration.
Molds, on the other hand, are multicellular microscopic fungi, typically characterized by the presence of hyphae filaments. The aggregation of the hyphae as a network constitutes the mycelium, visible to the eye.

Fungi: (singular fungus) are a kingdom of eukaryotic organisms. The fungi are heterotrophic organisms characterized by a chitinous cell wall, and in the majority of species, filamentous growth as multicellular hyphae forming a mycelium, some fungal species also grow as single cells. Sexual and asexual reproduction is via spores, often produced on specialized structures or in fruiting bodies. Yeasts, molds, and
mushrooms are examples of fungi. The discipline of biology devoted to the study of fungi is known as mycology.

Molds: (or moulds, see spelling differences) include all species of microscopic fungi that grow in the form of multicellular filaments, called hyphae.[1] In contrast, microscopic fungi that grow as single cells are called yeasts. A connected network of these tubular branching hyphae has the same DNA and is considered a single organism, referred to as a colony or in more technical terms a mycelium.

Le muffe non formano un raggruppamento tassonomico o filogenetico specifico, ma possono essere trovate nelle divisioni Zygomycota, Deuteromycota e Ascomycota. Anche se alcune muffe causano malattie o deterioramento degli alimenti, altre sono utili per il loro ruolo nella biodegradazione o nella produzione di vari alimenti, bevande, antibiotici ed enzimi.