A fungus is an organism in the kingdom Fungi. Fungi are extremely interesting organisms, sharing traits with both plants and animals which made them difficult for early scientists to classify. Originally, fungi were treated as plants, but this classification was found inadequate as people learned more about them, leading people to realize eventually that fungi really needed their own kingdom.
With over 80,000 known species within the fungus kingdom, it's hard to make sweeping generalizations about fungi. Several features characterize a fungus, however, including the lack of chlorophyll and vascular tissue. The lack of chlorophyll means that fungi cannot photosynthesize like plants do, while the lack of vascular tissue restricts the nutrients which a fungus can access, and the complexity it can achieve.
Fungi feed on organic material, secreting enzymes to break down material so that nutrients will be released in a way which allows the fungus to absorb them. Fungi can reproduce sexually and asexually in an assortment of ways, with some species sending out spores, while others asexually bud to produce clones of themselves. Along the way, many fungi help to break down leaf litter and other organic material, contributing to the slow cycle of recycling which keeps nature in balance.
Mushrooms, molds, smuts, yeasts, and lichens are all classified in the fungus kingdom. These organisms are incredibly diverse. Lichens, for example, form symbiotic living arrangements with bacteria, using the bacteria as an energy source. Yeasts can range from Candida, the yeast which causes thrush, to Saccharomyces cerevisiae, the yeast used by bakers to make bread. Molds like black mold can pose a threat to human health when they colonize structures, while smuts colonize plants such as corn, developing a parasitic relationship which humans can sometimes use to their advantage.
Fungi range from single-celled organisms to massive colonies of honey fungus which can cover impressive swaths of land. There are seven phyla in the fungus kingdom, each of which has distinctive defining characteristics. You might be familiar with representatives of several phyla: culinary mushrooms, for example, tend to belong to the Basidiomycota phylum, while many disease-causing yeasts, along with penicillin, come from the Ascomycota or “sac fungus” phylum.
Some fungi happen to be edible, with an occasional species being quite delicious. Others are incredibly toxic. Fungi can be used medicinally in a range of ways, and they can also pose medical problems when they colonize the human body. Fungi are also everywhere, from the air you breathe to the ground you walk on, and new species are always being uncovered.