ories in, calories out", but it is not an exact science. For the most part, we all need a specific number of calories to maintain, lose, or gain weight, but these numbers fluctuate between people due to genetics, age, metabolisms, activity levels, etc.
You can definitely experience some success by ingesting fewer calories, but the key is making every single one of your calories count. For example, you are far better off eating 200 calories of fruits and vegetables rather than 200 calories of candy. Not only will 200 calories of fruits and vegetables fill you up more, but they contain more useful 'stuff' in them such as vitamins and minerals.
To learn more about calories, you can read one of my past blogs here.
Photo - http://www.anthonyarroyodotcom.com/theabletoncookbook/wp-content/uploads/2011/10/Weighing-Scales-1.jpg