Fungi are some of the main organisms responsible for the decomposition of organic matter in any ecosystem. They look like plants that don't have chlorophyll, but they are not plants. They belong to their own kingdom. They produce microscopic spores (similar to seeds in plants).
There is a unique symbiosis between fungi and plants, in which fungi helps break down dead organic matter into nutrients. However, fungi can also enter plants through wounds, natural openings or by direct penetration of plants, carrying disease into the plant's system.
Yeast is a type of fungi that humans use to ferment beer and make bread. And, of course, many people love to eat mushrooms, another type of fungi.