I'm a bit of a science geek, so I'll try and make it non complicated without sounding patronising! I apologise if I do either, it's not intended!!!
Basically, a calorie is a unit of heat, or energy. 1lb of fat is roughly 3500 calories, or 3500 units of heat or energy. If a piece of chocolate has 65 calories in it, what it's saying is that your body will use up around 65 units of energy to process and digest that piece of chocolate. As you probably know, the recommended number of calories a day is 2000 for women and 2500 for men, because that's how many units of energy on average an active adult will use. And in exercise it's the number of extra units of heat that are burnt up through that activity. So we're losing weight because we're using up more energy than we're consuming. But your body can't just go well, no energy, shut down now! So instead it converts some of that fat and uses it as energy.
What basically happens is a metabolic process where enzymes break down the carbs into glucose and other sugars, the fats into... er.. glycerol (i remember now!! lol) and the proteins into amino acids.
Those glucose, glycerol and amino acid molecules then travel through your blood stream, rather than sitting as fat in one place. They're either used up along the way or in the final stage they react with oxygen and the energy within them is released.
Your body then uses that energy to 'fuel' all it's processes. Our muscles are basically like little engines that our bodies use to do anything, and they turn energy into motion. So every movement you make, that's fueled by that fat. In a way, it is that fat!
But I guess that doesn't completely answer the question does it! In a car, if it uses fuel, you get emissions through the exhaust. Your body doesn't quite do that. There is no emission. Just movement.
Does that make any sense at all?!