In simple terms, sauce is a flavored, thickened liquid designed to accompany food and enhance its flavor.
As many things culinary, the French are credited to have created the "sauce" . In the 19th century, Chef Antonin Carême devised a method that classified hundreds of sauces into one of five "mother sauces". The "mother sauces" include: espagnole - a brown stock-based sauce; veloute - a light stock-based sauce; bechamel - a simple white sauce; hollandaise - an egg based sauce flavored with lemon or vinegar and butter ; mayonnaise - an emulsion of eggs and oil, and vinaigrette - oil and vinegar combinations.
In addition to the "mother sauces" there are hundreds of sauces we use today, most of which are lighter in fat and calories ranging from soy sauce and worcestershire to tabasco, cocktail to hoisin.