Treat your calories like you would your checkbook. We all burn a certain number of calories each day (Resting Metabolic Rate). We can increase the number of calories that we burn by increasing our activity level (exercise).
Activity+RMR=Total Energy Expenditure
The Total Energy or Calories that we expend in a day vs. the energy/calories that we consume, determines whether we lose weight or gain weight. This does not make for a significant loss in 1 day, but it certainly adds up over time (this goes both ways).
Here is a link to a TEE calculator if you are interested: http://www.health-calc.com/diet/energy-expenditure-advanced
Be sure not to exaggerate your activity levels, as this can cause the numbers to be inaccurate.
Note: One should not be at more than a 300-500 calorie (kcal) deficit in a day (this applies to food intake, not calories burned through exercise). This deficit will allow for a ~1lb weight loss each week. The other pound should be lost via exercise.
*It is not recommended that a person lose more than 1-2lbs per week.