There are five "mother sauces" that make up most sauces according to French cooking. Béchamel, Espagnole, Hollandaise, Tomato, & Velouté. There are many variations, & most sauces are basically derived from these five.

Sauces can be "streched" or "thinned out" by adding more of a base ingredient, such as water or cream. If you see you have over-thickened a sauce, your sauce is not ruined. Just add more of the base ingedient to thin & re-season.