Answer:

An emulsion may be defined as a colloidal dispersion of two immiscible or partially immiscible liquids, in which one liquid acts as the dispersion medium and the other as the dispersed phase.

Depending on the nature of the dispersed phase, emulsions are broadly classified into two types

i) Oil in water type emulsion.
Oil acts as the dispersed phase water acts as a dispersion medium. e.g. milk.
ii) Water in oil type emulsion
water acts the dispersed phase and oil acts as the dispersion medium e.g. butter.
Emulsion are generally unstable and separate in two layer on standing.
Thus, to stabilise an emulsion small amount of certain other substance, called emulsifier or emulsifying agent are added. 
Protein are the principal emulsifying agent for oil in water types of emulsion.