Cotton voile and muslin are both lightweight, open-weave cotton fabrics - breathable, soft against the skin, and suited to projects where a fine, airy cloth is needed. While related, they have different qualities and uses.
Cotton voile is a fine, semi-sheer fabric with a higher thread count than muslin, giving it a smooth, slightly silky handle. It's used for blouses, dresses and lingerie where a lightweight, flowing cloth is needed, and works well as a lining beneath embroidered or sheer fabrics. It's also popular for lightweight curtains and window dressing.
Muslin has a looser, more open weave and a softer, more flexible hand. It's widely used for babywear, swaddle blankets and children's clothing, as well as for crafting, food preparation, and as a toile or test fabric for dressmaking patterns before cutting into your main cloth.