The difference between a rash and a red rash lies in the different ranges, shapes and sizes, and types of diseases included in both. 1. Different scope: Rash is a collective term for all the changes that occur in the skin, commonly including papules, erythema, wind clouds, blisters, vesicles, ulcers, scales, nodules, keloid scars, etc., which all belong to the signs of the rash. Rash is just one of the forms of rash. 2. Different shapes and sizes: the rash is mostly a single diameter of less than 0.5 centimeters above the skin, substantial damage, the top shape or pointed or round or irregular; while the size and shape of the rash varies, of which the small ones are mostly papules with a single area of 0.5 centimeters, and the large ones are mostly erythematous plaques or plaques, which can be often fused into a piece, and the shape is either higher than the skin surface, or sunken, or flat. 3. Disease type: Rash can appear in the development process of almost all skin diseases, while red rash is common in drug rash, measles, scarlet fever and other diseases. If the above symptoms appear on the skin, it is recommended to go to the hospital in time and choose the appropriate treatment under the guidance of the doctor.