The thermogenic (thermic) effect of food is the cost of digesting food and therefore the effect it has on your metabolism. A lot of people consider highly thermogenic foods to be "negative calorie" foods, and I'm sorry to say, but that's a myth. There are however foods with a significantly higher thermogenic effect than others, and while they aren't going to make you burn more than what you're ingesting, they will help you minimize your calorie intake by satisfying hunger cravings and by providing your body with tools that can aid your body's ability to burn fat.