A permanent emulsion is a combination of two ingredients that normally don't mix well, but they come together to form a new stable mixture that does not separate. A temporary emulsion is a blending of ingredients that eventually separate.
The most common emulsions are salad dressings. Simple dressings that are made from oil, water and seasonings are temporary emulsions, which is why they must be given a good shake before using so that the ingredients mix together. Dressings that use eggs or dairy products as a base, such as ranch, Caesar or blue cheese, are permanent emulsions that do not need shaking before use.
Mayonnaise is also a permanent emulsion. It is made by slowly incorporating egg yolks into oil with a touch of lemon juice or vinegar until they come together to form the final product.