Fungi are one of the four kingdoms in the Eukaryotic Domain. They used to be classified with plants since they look like plants but they are not autotrophic (meaning they do not make their own food like plants do). They are genetically closer to humans than they are to plants!
Fungi are saprotrophs meaning that they are decomposers. This makes them very important to the ecosystem as recyclers of dead material and waste matter. They and bacteria are the clean-up crew of Mother Nature. They dump digestive enzymes onto the food they are eating and then they absorb the digested meal through their cells. Think about this next time that you hear of someone having athlete's foot which is a fungus!
The body of fungi is made up of strands called hyphae. It takes a bunch of hypha together to make up the body of a fungus which is then called a mycelium. Often these hyphae grow through the medium being eaten and are rarely seen. What we usually see of the fungus is the fruiting body or reproductive structure of the fungus. For example, the green fuzzy stuff you see on bread is actually the reproductive structure of the bread mold not the actual body of the mold which is inside the bread.
There are five divisions or phyla of fungi most are classified by the reproductive structure the fungi have. Fungi reproduce by producing spores which can be released several feet into the atmosphere. These spores float around until they find a suitable spot for the fungus to grow. The types of fungi include Chytridiomycota which produce zoospores that have flagella and can move on their own. Zygomycota, which make zygospores that are created when two reproductive hyphae meet. Bread mold is a common Zygomycete. Glomeromycota reproduce asexually. Ascomycota make ascospores which are found in ascus or special sac structures. Truffles and yeasts are common examples of Ascomycetes. Basidiomycota are the last division which make basidiospores on special club-like structures called basidia. A common example of a basidiomycete is a mushroom.
Fungi also participate in two important symbiotic relationships. The first is called a lichen. Lichen are symbiotic or mutually beneficial relationship between a fungus and a green algae. The algae feed the fungus and the fungus gives the algae a place to grow.
The second important symbiotic relationship that fungi participate in is the relationship that they have with some plants. Some fungus grow on the roots of a plant feeding on the plant root cells, but they also give the plant water and minerals it would not otherwise attain. Some plants cannot survive without the corresponding fungus symbioant. So you can see that fungi play a very important role in our ecosystem! As well in our lives as they are used to make cheese, beer, bread, wine and penicillin.