Sign and icon are often used synonymously, but they have different meanings. A sign (lat. signum) is a general name for any understandable image, symbol, word or action that has meaning. A pictogram (lat. pictus + gr. gramma) is a graphic information sign, a small picture, often used in computer environments to represent folders, programs or other elements.
So, although an icon is a sign, not every sign is an icon. For example, a road sign is a sign, but not an icon. An icon is more defined as a specific type of sign that has a visual form and is used to convey information, especially in a digital environment.