If I eat 1,800 calories of pizza, soda and fried chicken, I will most likely gain weight, not maintain or lose. But if I eat 1,800 calories or steamed vegetables, complex carbohydrates and lean proteins, then I will probably lose weight. 20 calories of chicken breast and 20 calories of a calzone are not the same thing!
Calories come in three forms; fats, carbohydrates, and protein. You need all three forms of calories to support a healthy body. One gram of fat contains nine calories, and one gram of carbohydrates contains four. One gram of protein also contains four calories. Let's look at these three calorie sources more in-depth. If you have a pure fat, like oil, you can assume that all of the calories in that food source come from fat. Since most of what we eat is a combination of fats, carbs and proteins, it's always a good idea to really get to know what you are eating, especially if you are trying to lose weight or put on muscle.
Something else to consider when planning out your calories, is how often you are eating. If I eat 1,800 calories all in one meal rather than spacing out my 1,800 calories throughout the day, my body will store of my that food as fat. Eating regular, small meals keeps your metabolism high. You will burn through your meals more quickly, with less of a chance for your body to store what you just ate as fat. Eating only one large meal everyday signals the body to store everything you just ate, because it isn't expecting to be fed again for another 24 hours. Keep meals small and eat every two or three hours and you will see a big change not only in your metabolism, but also in your body composition.