Emulsifiers hold two ingredients together that wouldn't normally mix. This can be either something like wax or the physical action of shaking the bottle. Synthetic emulsifiers are usually petroleum/hydrocarbon derivaties and can be allergens. Natural emulsifiers are obtained from various nuts, berries and leaves.
Surface active agents used to facilitate or increase the dispersion of one liquid in another when one is not miscible in the other.
Additives that allow oils to be blended with water, overcoming their natural reluntance to mix.