The singularly form is fungus. Organisms in the Kingdom fungi are eukaryotes. This means that the have eukaryotic cells, those with a nucleus and membrane-bound organelles. Like plant cells, the cells of fungi have cell walls. However, the cell walls are composed of chitin, not cellulose.
Fungi are unlike algae in that they are heterotrophic.
This means that they rely on food from their environment to obtain energy. Fungi, like animals do not carry out photosynthesis. Fungi release digestive enzymes into their food and digest it externally. They absorb the food molecules that result from the external digestion. Fungi are able to effectively digest food externally because, in all fungi but yeast, the organism consists of long, thread-like structures called hyphae which surround and grow into the food source, living or dead.