To be clear, insect repellents are not pesticides or insecticides. In short, they do not kill insects.
All repellents, whether they are synthetic or natural, work by discouraging insects from landing and climbing on treated surfaces, and therefore, biting treated skin.
Repellents are actually made of various chemical composites as well as natural oils derived from plants. Common active ingredients in synthetic insect repellents are DEET (N,N-diethyl-m-toluamide) and Picaridin (KBR 3023) (CDC, 2008). Natural repellents are made of oil of lemon eucalyptus, citronella oil, lemongrass and geranium.
Synthetic repellents last longer. However, you need to reapply both types of insect repellent if you perspire or get wet.