A common misconception in dieting is all calories are the same. However, all calories are certainly not the same when trying to optimize your body and how many calories you consume will determine you losing weight or gaining weight, as will the quality of the calories you consume.
For more details on why not all calories are the same, this video describes how calories are different. It highlights some important aspects of calories, such as their macronutrients and vitamin and mineral content. It also explains how weight loss works in relation to calories, and how you can optimize your weight loss goals.