Moulds / Molds
Moulds (sometimes spelled molds) are microscopic fungi whose spores are everywhere. These spores can germinate quickly and colonize new food substrates (e.g., bread, cheese, peaches) very quickly. They may be white, pink, green, blue, grey or black, and often look like spreading fuzzy spots. They usually grow only where it is damp.
There are two major kinds of mould, which belong to two very different groups of fungi.
(1) The first and most numerous group are the Hyphomycetes, mostly asexual phases of a Class called Ascomycetes. There are many thousands of different moulds in the Hyphomycetes.
(2) The second group often grow very fast, and are asexual phases of the Class Zygomycetes . There are several hundred of these moulds, and most can grow only in very damp places.
Both kinds specialize in producing very large numbers of spores with minimum expenditure of energy, since the spore bearing structures are always microscopic.
The structure of the two groups is rather different, and it is not hard to tell them apart, as you will find if you browse the two pages linked above.
Much more detailed accounts can be found in 11 chapters of the book The Fifth Kingdom