Calories are units of energy that measure the amount of energy in food. Most diets take calories into account and simply reducing calories will promote weight loss. It isn't that simple though, if your body was a furnace like a wood stove then yeah it would work, but simply looking at all the different diets around will tell you that it is not the full answer. Your body is not a furnace it doesn't burn the food to create energy it uses different chemical reactions to change the different substances into something else and by doing so releases energy.
Calories were introduced years ago, as far back as I can remember they were incorporated into every diet. I remember back in high school my teacher showed us that placing a food item in a flame will produce different colours. The colours represented the different chemicals in the food and the different colours indicated the amount of energy the food has. This is the premise that most people think about food. Throw the food in a fire and its burns releasing energy. Just like throwing wood in a fire and just like throwing coal in a steam engine and the car consuming gasoline. Unfortunately the body works differently it simply does not "burn" the fuel. If it did people would be able control their diet simply by adjusting how many calories they eat. The simple fact that some people are able to do so and others not. There's a point that counting calories stop working and becomes almost impossible to lose the weight. Then they work out and lose a little more by "burning" the fuel. Well, the body does not work that way it doesn't "burn" fuel, it changes it in a chemical reaction into something else and it's the something else it changes it into that creates the problem.
Take a carrot for example, the orange outer skin and tuft of green stem. It has energy, throw it in a fire and it doesn't burn well, but it burns. For arguments sake it has energy from the carbohydrates locked in the cells and the fibre that holds it all together. With some vitamins and minerals also locked inside. So in other words it has nutritional value, the body can use almost everything in a carrot and converts the carrot into its base components releasing energy and uses those base components to build and rebuild the body expending energy by doing so. Some other elements of the carrot are broken down again and again energy is released. So by the breaking down the carrot into building blocks energy is released. By using those building blocks to make cells grow in our body energy is used. In turn those cells breakdown the remainder of food pieces and releasing more energy. Almost every piece of a carrot is used by the body and converts into something else thus releasing some energy. Not the same as burning it.
Take for example the diet drinks that have zero calories. You really think that it is zero? No, it doesn't. That diet drink has chemicals in place of food. Chemicals that mimic the taste of the real thing. Everything that you eat, the body has to try and use it, if it can't use it converts into something that the body can release through the kidney and the bowels. It is the liver that does the heavy lifting of converting chemicals and food into what we can use and throws away which is just garbage. Just imagine a person working on a line in a factory who's sole job is to sort stuff we can use and the rest is just garbage and throws it away. If you could see the amount of garbage that is thrown away by our bodies you would definitely change what you eat and how you eat it. The conversion of the garbage material has to be broken down again and again and again into something that has be thrown out or used. The by-products of the garbage is inflammation, purines, uric acid and triglycerides. Triglycerides is also known as fat. So the zero calorie drink creates fat and inflammation. I thought it was zero, Nope the body tries to use everything it can to survive but when it can't it throws it out. Since the body can use fat it makes it but at a cost and if it can't use it right away it stores it as fat where it can. Mostly men around the mid section and women in and around the hips. Don't get me wrong we need fat in our diets in order to have a balanced diet but it's the amount of garbage we eat that creates the problem. Zero calories does not mean zero energy and it is hidden in everything that we eat, we don't see it, but it is there.
It is for this reason that you need to eat more carrots or vegetables vs a chocolate bar. You need more vegetables to obtain the same amount energy in a chocolate bar. So why not just eat the chocolate bar? You can! You'll just be able eat 2 chocolate bars a day and then that's it and exercise 30 mins to an hour a day. I'm sure you'll agree, that is not possible to do and you will be right. A calorie is not a calorie and not every calorie in food is created equal, so much for equal rights.
I can guarantee that if you eat just barely the amount of energy your body needs (a.k.a. Basal metabolic rate) at a fast food restaurant you will gain weight, guaranteed.
The problem lies with the quality of the food and the quantity of the garbage we eat. It boils down to that simple premise.