Moulds are filamentous (thread-like) fungi. A single filament is called a hypha. The hyphae branch as they grow forming a network called a mycelium.
Each hypha grows from the tip and divides repeatedly along its length. The hyphae penetrate their food source (usually dead, but sometimes living, plant and animal matter). They excrete enzymes which break down the complex organic molecules into simpler substances. The soluble nutrients pass through the cell wall and membrane, enabling the fungus to grow.
In most moulds the hyphae are divided by cross walls called septa which help to make filaments rigid but also control nutrient flow.
Moulds can grow in dry and acid conditions and can tolerate a wide range of temperatures. These fungi produce airborne spores.
Examples of moulds are: Penicillium, Mucor, Aspergillus.