Fungi are important organisms that belong to their own kingdom, completely separate from plants and animals. A hugely diverse group of great economic importance (such as Penicillium, yeasts, crop-destroying rusts and smuts, and much more), fungi remain vastly under-studied compared to plants. It is estimated that there may be around 700,000 to 5 million species of fungi in the world. They are more than six times as diverse as flowering plants. Yet only about 100,000 species have so far been described.
There are two different terms ‘fungi and Fungi’ and these two has different meaning. The lower case 'fungi' is a general word that refers to organisms that all look and act the same, but are not all related. This group is artificial and includes moulds, yeasts, mushrooms, slime moulds, and water moulds (like Phytophthora, the cause of the Irish potato famine and Sudden Oak Death). On the other hand, 'Fungi', with a capital 'F', refers to the evolutionary group that includes most of the best known 'fungi': moulds, yeasts, and mushrooms, but not slime moulds or water moulds. Because all of these organisms superficially resemble each other and all do similar things, they were grouped together within the lower plants, including mosses, liverworts, and ferns, for a very long time.
Fungi are tremendously important to human society and the planet we live on. They provide fundamental products including foods, medicines, and enzymes important to industry. They are also the unsung heroes of nearly all terrestrial ecosystems, hidden from view but inseparable from the processes that sustain life on the planet.