Mould (or mold) is a type of fungus. Mould grows from spores, which float around in the air. There are thousands of different kinds. They are often seen in wet places. Molds include all species of microscopic fungi. In contrast, microscopic fungi that grow as single cells are called yeasts.
Some molds cause disease or food spoilage. Sometimes people use them to make some kinds of cheese and antibiotics. One special kind of mold can be used to make penicillin, a common antibiotic.