Fungi (singular fungus) are a kingdom of eukaryotic organisms that are distinguished by being icky and gross. While in some ways similar to plants, in particular because of their root systems (called hyphae), fungi are in fact heterotrophic organisms which means that they grow out of yucky stuff like old food, rotting wood, and smelly feet because they cannot photosynthesize like plants. Yeasts, molds, and mushrooms are examples of fungi. The study of fungi, in addition to being absolutely disgusting, is called mycology.
General features and nastiness
Different types of fungi are adapted to live in nearly any environment, but they seem to have a creepy preference for refrigerators and all kinds of gross-smelling things. There are an estimated 1.5 million fungal species in the world, which is way too many considering that even one is more nauseating than anything needs to be.
Many species of fungi are cultivated by humans for various purposes. Some freaks actually like to eat mushrooms, even though it's totally obvious that mushrooms aren't meant to be eaten, even the ones that aren't poisonous. Some varieties of cheese are made with a mold fungus to give it a particular texture and flavor–one of supreme nastiness. Penicillin is derived from a fungus, as are many other antibiotics, but frankly it would be less icky to just deal with the bacteria; at least we can't see those.
While often ignored or unseen in their natural environment, fungi are everywhere, creeping all over stuff that hasn't been properly sprayed down with bleach. They are critical decomposers of organic matter, but there's no goddamn way I'm going to let them grow on my corpse when I kick the bucket.