A rash is a noticeable change in the texture or color of the skin. The skin may become itchy, bumpy, chapped, scaly, or otherwise irritated. Rashes are caused by a wide range of conditions, including allergies, medication, cosmetics, and various diseases.he skin may become itchy, bumpy, chapped, scaly, or otherwise irritated. Rashes are caused by a wide range of conditions, including allergies, medication, cosmetics, and various diseases.