Mildew is mould. That is the whole technical answer, and it is unsatisfying because the question people are really asking is whether what they are looking at is the mild kind or the serious kind. The useful distinction is not taxonomic at all: it is between thin surface growth on a finish that is periodically damp, and growth in or behind a material that is wet. The first is a ventilation problem; the second is a building problem.
What the words actually mean
Mildew is a common name, not a biological category. In ordinary use it means thin, flat, powdery surface growth, usually white or grey, on a damp finish: bathroom grout, a window reveal, a shower ceiling, fabric left damp. Mould is the general term for the same kingdom of organisms including the ones that grow into and through materials. Every mildew is a mould. Not every mould presents as mildew, and that presentation is what people are describing when they use the word.
What does mildew look like, and where does it turn up
What does mildew look like in practice: flat rather than raised, powdery or dusty rather than fuzzy, usually pale, sitting on top of the surface and wiping away to leave a sound substrate. Mildew on wood reads as a grey or white bloom that abrades off, leaving timber that is still firm. Orange mildew and the pink films in showers are frequently bacterial rather than fungal. Raised, fuzzy, dark, spreading or returning growth is telling you the substrate itself is wet, which is a different job.
Mildew vs black mold, and toxic black mold vs black mold
The phrase toxic black mold generally refers to one dark coloured species associated with toxin production under some conditions, and non toxic black mold is the everyday way of asking whether a dark colony is that one. Public health guidance is consistent and worth repeating here: indoor growth should be removed and the moisture corrected regardless of species, because identifying it rarely changes what has to be done and sampling is slow and expensive. Sensitised and immunocompromised people react more to any of it.
When the distinction changes what you do
Only once. Thin surface growth on a sound dry finish is cleaned, and the ventilation or the condensation that fed it is corrected, and that is a household job. Growth that returns within weeks, that sits on a substrate reading wet on a meter, that is in a cavity, or that covers a large area, is a remediation with containment and a moisture investigation behind it. The word you use for it is irrelevant; the meter reading is not.
Questions people ask about difference between mold and mildew
What is the difference between mold and mildew?
Mildew is a common name for thin surface growth on a damp finish. It is mould. The useful distinction is between surface growth on a dry substrate and growth in or behind a material that is wet.
Is mildew mold?
Yes. Mildew is not a separate organism or a milder category; it is a description of how some mould presents on a surface.
Is toxic black mold different from non toxic black mold?
The phrase usually points at one dark species associated with toxin production. Guidance treats all indoor growth the same way, so the label rarely changes the work.
Can I just wipe mildew off?
If the substrate under it is sound and dry, and the ventilation or condensation is corrected, yes. If it returns in the same place within weeks, the material is wet and it is not a cleaning job.