Just what is American "cheese" anyway?

Home from work and desperate for a snack. Nothing savory in the cabinet. Wait! there’s some American “cheese” left in the fridge from….when? Grilled a sandwich; tasted great in that salty, creamy American “cheesey” way. So – here’s the question (wait for it.) I can’t remember when I bought that stuff; I think it was before my kid left home last spring. What the hell is it really made of?