American cheese is just normal, cheddar style cheese but with the addition of some sodium citrate.
Sodium citrate is a fun little food chemical that give the cheese a slight citrus bite, but more importantly acts as an emulsifier. It keeps the oil and water inside the cheese bonded together. This means the cheese melts and then never becomes greasy.
You can abuse this. You can make a super creamy cheese sauce for a higher brow mac and cheese by using some shredded cheese, a couple slices of American, and a few splashes of the pasta water.