the addition of multiple other toppings for mac n cheese is huge - onion, lobster, shrimp, truffle, ketchup or vegetables!
a roux is a thickening agent used to make sauces. It is made when cooking fat (butter in this case) and flour together.
some recipes call for Mac n Cheese to be baked in the oven and other just on a stove top. Either way works just fine. The stove top method I find offers a creamier version where the baked one gives you a stringy cheese affect. It's personal and time dictated too!