Muslins are open weave fabrics that are used for craftwork, baby products, cheesecloth, straining and sometimes drapes. Muslin should be washed before use for straining as it it still has starch in it.
Voile is similar but tighter woven from fine yarn, so more floaty and has fashion use as well as craft and drapery. See also Batiste which is still very fine but not quite so sheer. Batiste
Muslin doublecloth is two fine layers joined together, with a square woven pattern