Mold is a fungus that grows in moist areas. Mold has a distinctly unpleasant, musty smell like rotten wood, wet socks, or soggy paper.
Mildew refers to a certain type of mold and is often used to refer to mold growth. Mildew often lives on shower stalls, windowsills, and other places where moisture levels are high.
Dust is dry and has a earthy smell since dust typically consists of dirt, pollen, and soot brought in from outside as well as hair, pet hair, clothing/carpeting lint.