Surface active substances and agents, also called surfactants, are dissolved in a liquid to make them blend easier with other substances, such as oil. They work to reduce surface tension between the two liquids. This use of emulsification is often used in textile making, where a surface active agent can be added to a pigment before it is mixed in water to ensure the finished dye solution properly permeates the fabric.
Detergents are emulsifiers. The water-soluble cleansing agents can dissolve oil and help residues mix with water. It would be almost impossible to clean without them. Detergents are mixed into water to permeate grease and dirt on clothes, dishes, tiles and floors. The oil, once dissolved or mixed into the water solution, can be rinsed away cleanly.
There are many emulsifiers that can be found in nature. These substances contain both hydrophilic (water-loving) and hydrophobic (water-hating) molecules. Examples of these include egg yolk, which contains both phospholipids and cholesterol; lanolin, or wool oil; starch, which reduces surface tension like surface active agents; and lactic acid esters, which are found in milk.
Powdered emulsifying agents, such as diacetyl tartaric, can be used to soften proteins found in foods. In this case, diacetyl tartaric relaxes the gluten in wheat flour so the bread can form more evenly shaped bubbles as it is baking. While this might seem quite different than mixing oil and vinegar to make salad dressing, this emulsifying agent is using the same principle -- making something hydrophobic mix more evenly with water -- to create a different result.