The fungi are a kingdom in the eukaryote domain. Fungi are important decomposers in all ecosystems because they can break down a wide variety of organic matter. Lignin, a component of wood, would not get broken down without fungal decomposers, so the nutrients would not be recycled.
All fungi are heterotrophs. Instead of having a traditional stomach, many of them secrete enzymes into the soil near them to break down organic matter and then absorb the nutrients. Like plants, fungi have cell walls, but they are made of chitin instead of cellulose. Most fungi are multicellular organisms, but some common forms such as yeast are single celled. Mushrooms are a common example of multicellular fungi that humans use regularly.