The taxonomic kingdoms of life are the biologist's way of classifying all organisms on earth. Among them are the well known groups such as plants and animals, and some divisions that will take you back to high school biology. There are a total of seven kingdoms in the taxonomic tree. It is important to note that while the taxonomic tree is as simple as depicted in the diagram, the phylogenetic tree of life, in which placement is determined by genetic differences, contains over 20 kingdoms. For the sake of sanity and simplicity, I will refer to the taxonomic tree.
The largest dividing criteria, which divides even the kingdoms themselves, is the prokaryotic / eukaryotic line. The easiest way to describe this division is to say that eukaryotes have intercellular organs and a nucleus, while prokaryotes do not. All bacteria and archaebacteria are prokaryotic.
Kingdom fungi is often called the Fifth Kingdom; in the days before molecular biology and electron microscopes, all life was classified as either animal, plant, protist or bacteria. Eventually, fungi were determined to be different from plants in significant ways, and the fifth kingdom, kingdom Fungi, was established.
Below are short descriptions of all the kingdoms:
Bacteria: Bacteria are defined as being single celled with no nucleus and no intracellular organs. Even though bacteria are single celled individuals, most can not survive alone and must form colonies called biofilms to be effective. Bacteria can metabolize nearly every carbon based molecule, in addition to using sulfur, ammonia, nitrate, and iron as energy sources. Cyanobacteria use a special form of photosynthesis and therefore create their own food. Bacteria are an incredibly adaptable and diverse kingdom of life that is present on pretty much every surface on earth and all the spaces inbetween.
Archaea: While archaea resemble eubacteria in many ways, they are genetically and evolutionarily very different. The archaea are adapted to succeed in environments that are too harsh or lacking in typical nutrients for other organisms to survive in. Their niche are in thermal vents, geothermal pools, salt flats, and soda lakes. They can also survive in milder environments but do so by utilizing atypical nutrients for which there is no other competition.
Protista: Protists are single celled eukaryotes. There is great diversity among the protists, and a significant portion of the protist kingdom is photosynthetic. Amoeba, paramecium, diatoms and algae are examples. In aquatic environments, photosynthetic protists are the primary suppliers in many food webs.
Plantae: Depending on who you speak to, the simple descripton of plants is that they are multicellular photosynthetic organisms. Also included are non-photosynthetic organisms such as indian pipes and orchids, both of which are considered to have adapted from photosynthetic ancestors. Some classification systems also include single celled green algae in the plant kingdom because of its genetic and molecular similarity. Because plants are autotrophs (produce their own food) they are the primary suppliers in most land based food webs. Plants are the primary food source for most fungi, especially when it comes to macrofungi such as mushrooms.
Animalia: For the most part, animals are motile, multicellular eukaryotes that do not produce their own food so must consume other organisms or the remains / byproducts of other organisms to survive. There is tremendous diversity among the animals, not only in their genetics but also their appearance (morphology). While most plants have leaves, stems, and are nearly all colored in shades of green, animals are diverse in nearly every imaginable aspect within the kingdom criteria. Sponges, insects, worms, molluscs, birds, mammals... it is estimated that there are over 9 million species of animals on earth, many of which are insects.
Fungi: This is the kingdom that you came here to learn about. Kingdom fungi is the other multicellular kingdom that is defined by obtaining its food from an outside source. In the earlier years (decades, centuries) of biological classification, fungi were combined with plants; probably because they were immobile things that tended to appear in the company of other plants after rain. But in time, biologists got wise to how fungi are significantly different from all other forms of life on earth. First off, fungi aren't motile(a few exceptions exist), so they can't be animals. Also, fungi can't produce their own food, so they can't be plants. On top of this, all fungi digest their food outside of their body by secreting enzymes. While a weak argument could be made against spiders, all animals and even all non-photosynthetic protists must take in their food to digest it. There are plenty more differences that make fungi unique, including how they produce spores, and the threadlike nature of their body (hyphal form). Another very interesting feature of fungi is that, unlike all animals and most plants, no fungi are embryonic. Their entire structure from germination to reproduction is created on the fly. Fungi are an especially diverse lot, second only to the insects in estimated number of species.
Chromista: This is the island of misfit toys on the taxonomic tree. All members of kingdom Chromista have properties that prevent them from belonging to another kingdom. The properties are typically genetic and molecular; such as having significantly different cell membrane chemistry, or a unique form of chlorophyll. It may not seem like much, but these differences indicate that these organisms evolved along separate pathways from those belonging to the other kingdoms. Some of the organisms that fall into kingdom Chromista are important to mycologists. Slime molds and water molds fall into this category, as even though they share some features with kingdom Fungi, they are too different in other aspects to be considered true fungi.