In the 19th century, the chef Antonin Carême classified sauces into four families, each of which was based on a mother sauce. Carême's four mother sauces were:
* Allemande is based on stock with egg yolk & lemon juice
* Béchamel is based on flour and milk
* Espagnole is based on brown stock, beef etc.
* Velouté is based on a light broth, fish, chicken or veal.
In the early 20th century, the chef Auguste Escoffier updated the classification, replacing sauce Allemande with egg-based emulsions (Hollandaise and mayonnaise), and adding tomate. Escoffier's schema is still taught to chefs today:
* Béchamel
* Espagnole
* Hollandaise
* Mayonnaise is not a sauce. Grrr... So this is not one of the mother sauces.
* Tomato sauce
* Velouté
Those sauces are called "mother sauces" because most other sauces can be derived from them. For example, Mornay sauce is a cheese sauce based on bechamel.
http://en.wikipedia.org/wiki/Sauce