Can you tell me what are pimples or allergic rashes, what's the difference?

Different diseases. Pimples is a layperson's name for the lesions of acne. There are several kinds of allergic rashes several: Allergic urticaria (hives); angioedema (swelling of the lips, eyelids, of the skin, hands, etc); atopic dermatitis (allergic eczema) affecting mainly babies and children, usually due to food or environmental allergy; allergic contact dermatitis (allergy to nickel, perfumes, soaps etc.)