The two words are common usage words not scientific words so a precise distinction cannot be made. All molds and mildews are types of fungus which makes them cousins (so to speak) of the larger fungi like mushrooms and the smaller unicellular fungi like yeasts. The distinguishing thing about mold and mildew is that they grow in filaments in a more or less flat manner.

Mold is the more inclusive word, all mildews are molds but not all molds are mildews. Molds include the organisms that create blue cheese for instance, and those that ruin bread but make the drug penicillin.

The word mildew is usually used either to describe the mold growing on flat surfaces like walls and counters that have been left damp or else to describe two families of plant diseases.

There is only a distinction between how the words are used, there is no defining difference as there would be between different families of living thing.

My favorite mold grows in gorgonzola, I don't like any mildews.

Mold and mildew have many similar characteristics, but they are different types of fungi, and are often different in color and texture. Mold and mildew are often mentioned together as they can both grow in many of the same moist and warm locations. However, mildew is often found more often than mold in showers, paper and fabrics and mold is usually found more often than mildew in foods. They can sometimes be difficult to tell apart as they both spread out from spores. Mold is often black, green, red or blue in color while mildew is usually gray or white.

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