Mushrooms, molds, yeasts, truffles; these all belong to the group of organisms known as fungi. They are not plants or animals or bacteria (although many are microscopic), they are – fungi. There are about 70,000 described species of fungi. 

This is however only the tip of the iceberg; most fungi are still undiscovered and undescribed and the actual number probably exceeds 1.5 million! (Hawksworth, 1991; Hawksworth et al., 1995). This makes them the second most diverse group of eukaryotic organisms on earth after the animals. Linnaeus, the founder of modern classification, considered  fungi to be plants (of a peculiar type) and included them in the plant kingdom. However, based on various lines of evidence including DNA, fungi are thought to have evolved at least 900 million years ago from an ancestral organism that also gave rise to animals.  So fungi are more closely related to animals than they are to plants, and have been very successful in an evolutionary sense. (To get an idea of the diversity of fungi and their relationship to other organisms, check out the Tree of Life website http://tolweb.org/tree/ ). The study of fungi is a discipline in itself known as mycology, but fungi are the subject of study by many other types of scientists, including plant pathologists, microbiologists, biotechnologists, medical mycologists, soil scientists and ecologists.