A fungus (/ˈfʌŋɡəs/; plural: fungi) is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants and animals. One major difference is that fungal cells have cell walls that contain chitin, unlike the cell walls of plants and some protists, which contain cellulose. These and other differences show that the fungi form a single group of related organisms, named the Eumycota. The discipline of biology devoted to the study of fungi is known as mycology. Genetic studies have shown that fungi are more closely related to animals than to plants.

Abundant worldwide, most fungi are inconspicuous because of the small size of their structures, and their cryptic lifestyles in soil, on dead matter, and as symbionts of plants, animals, or other fungi. They may become noticeable when fruiting, either as mushrooms or molds. Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange. However, little is known of the true biodiversity of Kingdom Fungi, which has been estimated at 1.5 million to 5 million species. Phylogenetic studies published in the last decade have helped reshape the classification of Kingdom Fungi, which is divided into one subkingdom, seven phyla, and ten subphyla. A group of all the fungi present in a particular area or geographic region is known as mycobiota. (see more...)