Ok - we've all heard of them, and some of us live out our lives by obsesivly counting them - but what exactly are they? A calorie is simply a unit of measurement of energy. There, that was worth waiting for! Here's the scientific bit - brace yourself "a calorie is the amount of energy, or heat it takes to raise the temperature of 1kg of water by 1C. One calorie is also equal to 4.184 joules, a common unit of energy in science."
We'll all sleep better now knowing that! But how is it relevent to us, the counters of calories, the user- uppers of energy, the fat people with thin people trapped inside them? Well think of it as money and a form of banking, the number of calories (money) you consume (bank deposit) is another way of describing the amount of energy your body gets from eating and drinking. Money in, money out, calories consumed, calories used.
Calories are essential for energy, however eating too many and not burning them off means they end up stored as our favorite friend - fat! So using (withdrawing from the bank) calories via movement is how all this is relevant to us. If you burn the same amount of calories that you consume, you should stay the same weight, but if you up your anti, the results can be weight loss.