The basic function of emulsifier is to make water and oil mix. If a product has both oil based and water based ingredients, it tends to separate naturally. Emulsifiers are a group of food additives that holds them together

Emulsifiers are commonly used in ice creams, Nut butters, Mayonnaise, Sauces, Breads, cakes, etc..

Any ingredient that is coded as and from E400 to E499 refers to the group of emulsifiers, stabilisers and thickeners.
Stabilisers are another set of food additives which are used to maintain the state of emulsification over a period of time.

Apart from the chemical emulsifying agents, emulsifiers are also derived from natural sources like plant based source and animal based source.
Soy lecithin and pectin are common emulsifiers that are derived from plant based sources. Gelatine is commonly used animal based emulsifier. Also it’s quite difficult to trace the origin of the emulsifier once it is derived from its source unless it is disclosed by the manufacturer of the product.

Milk, butter, egg yolk, mustard, flax seed are some of the natural emulsifiers that are used in day to day cooking.