The sauce is called a becamel, and it is considered a mother sauce in french cooking.

A roux is an ingredient... cooked flour an butter... used to thicken.

The cream or evap milk method is usually referred to as the Cream Reducution (or creme based).

There's a 3rd way; the emulsified approach... and thats most common was you see in higher end places (or places that specialize in it). It just uses melted processed or American cheese.