You need a certain number of calories each day based on the amount of energy you burn. And the amount of energy you burn is determined by your metabolism, your weight, and your activity level.
Regardless of your diet, if you take in more calories than you need, you gain weight. In fact, if you consume 3,500 more calories per week than you need, you gain 1 pound. If you want to lose 1 pound, the reverse is also true, you need to consume 3,500 fewer calories per week or burn 3,500 more calories. If you reduce your daily calories by just 500 each day (the max you should eliminate from your daily intake), you can lose 1 pound in one week.
Source: Well-Being For Dummies published by John Wiley & Sons, Inc.