Fungi refers to any member of the animal kingdom Fungi; these organisms are heterotrophs that feed on things already present in their environment. Fungi is the plural form of the word fungus.
A heterotroph is an organism that uses organic carbon as an energy source. Fungi obtain organic carbon through the process of decomposition. They break down organisms that are dead or decaying, performing a valuable role for their ecosystem. Without decomposition, dead organisms would continue to accumulate until they took over the ecosystem.
Fungi use a process known as extracellular digestion to perform decomposition. They secrete an enzyme onto potential food and absorb the food through passive diffusion. The food is then stored as glycogen for later use. A fungus reaches its food by growing larger so that it can reach the food source.
With the exception of yeast, fungi are multicellular organisms that are composed of filaments known as hyphae. They have cell walls that are made up of glucans and chitin. Chitin is the same material that is used to make up the exoskeletons of insects, thus providing the fungi with a relatively strong cell wall.
Fungi are found all over the world in every single type of habitat imaginable. Their widespread presence is indicative of their ecological importance and hardiness as a species. They can survive on land, in water and in ground sediment.