EPOC stands for Excess Post Exercise Oxygen Consumption and is the scientific term for the afterburn effect of prolonged, intense exercise. When you work out for a long period of time, the muscles begin to burn and you feel out of breath. Why? The muscles fill up with lactic acid (the burning feeling) and the body's oxygen stores become depleted.
When the body is trying to replenish the oxygen that is used during exercise it helps to burn calories long after the workout has ended. High intensity training sessions like boot camp classes forces the body to work harder to build it's oxygen stores back up and can take up to 16 to 24 hours post-workout.
What is the result? When you do more intense workouts as opposed to steady state endurance exercise it may burn the same calories but working at a harder intensity takes more time to replenish the oxygen stores. This means your metabolism works harder and longer? Good thing for weight loss!