Mildew refers to certain kinds of mold or fungus.  The term mildew is often used generically to refer to mold growth, usually with a flat growth habit.

Molds include all species of microscopic fungi that grow in the form of multicellular filaments, called hyphae.  Molds can thrive on any organic matter, including clothing, leather, paper, and the ceilings, walls and floors of homes with moisture management problems. Mildew often lives on shower walls, windowsills, and other places where moisture levels are high. There are many species of molds. In unaired places, such as basements, they can produce a strong musty odor.

Learn more about mold at www.epa.gov/mold.