Fungi can be single-celled, like yeast, or multicellular, like mushrooms, with structures made of thread-like filaments called hyphae. These filaments form networks known as mycelium, which spread through soil, wood, or other organic matter to absorb nutrients. Some fungi form symbiotic relationships with plants, helping them absorb water and minerals, while others can be parasitic and cause disease in plants, animals, or humans.

Fungi have many important uses in medicine, food, and industry. They are the source of antibiotics like penicillin, are essential in baking and brewing, and are used in biotechnology and environmental cleanup. At the same time, some fungi produce toxins or cause infections that can be harmful to health. Fungi play a diverse and vital role in natural systems and human life.