The taxonomic categories in descending order are: Kingdom, Phylum (Division in plants), Class, Order, Family, Genus and Species. Species is the lowest category and represents a group of individual organisms with fundamental similarities that can interbreed. As we move up the hierarchy, the number of common characteristics decreases while the number of organisms in the group increases.

R. H. Whittaker (1969) proposed five kingdoms: Monera, Protista, Fungi, Plantae and Animalia. The main criteria used were: cell structure (prokaryotic or eukaryotic), body organisation (unicellular or multicellular), mode of nutrition (autotrophic or heterotrophic), reproduction and phylogenetic relationships.

Fungi are eukaryotic, mostly multicellular (except yeast), heterotrophic organisms with a cell wall made of chitin. Their body is made of thread-like hyphae forming a mycelium. Nutrition is saprophytic (on dead organic matter), parasitic or symbiotic; they store food as glycogen. The four classes are Phycomycetes (e.g. Rhizopus, Mucor), Ascomycetes (e.g. Penicillium, yeast), Basidiomycetes (e.g. mushroom, Puccinia) and Deuteromycetes (imperfect fungi, e.g. Alternaria).