White sauce is dairy based, but what is it exactly? What are the main ingredients? Is it thickened somehow or not? How thick should it be? Should it contain herbs or oil?
A white sauce, also known as Béchamel sauce, is butter, flour, and milk. The flour and butter are cooked together to make a roux, and that thickened base is thinned out with liquid (in this case, dairy) to make a sauce.
It can have different thicknesses depending on what you want to use it for — varying the ratio of roux to milk results in a thin, medium, or thick white sauce.
Béchamel is a "mother sauce," meaning it is the basis for many other recipes: you can add cheese, herbs, seasonings, etc., to create a variety of sauces. However, a basic white sauce just has salt and pepper (white pepper if you want to avoid dark flecks in the sauce).
Suggestions for the best way to make the sauce can be found in this related Q&A: What's the best way to make Béchamel sauce?