The Super Melters: Process Cheese
Process cheeses, including "American" cheese, are cheese products designed to be extremely meltable and nearly impossible to break. This is accomplished by starting with real cheese (usually a young cheddar-style), and blending it with extra milk (for the added water), extra milk protein micelles (for the strong protein structure), and some form of chemical salt that helps prevent proteins from tightening. Because American-style cheeses have so much excess liquid added to them, they have extremely low melting points, making for extra-gooey grilled cheese sandwiches or cheeseburgers. This meltability, of course, comes at the cost of less intense flavor.
A lot of stuff we call cheese it really process cheese.