When I was growing up, it was common belief that celery was so low in calories and so difficult to chew & digest that it was actually negative calories. The theory was that you could eat a ton of celery and it would counteract other things you had digested. It is true that one cup of celery contains a mere 18 calories, and all food that you consume takes some energy to digest. That does not mean that eating celery will reduce the 400 calories of ice cream down to 350. Celery is a great choice for something to munch on though.
This belief of negative calorie foods is still being propagated today, and the lists has grown to include many other vegetables and some fruits. The thing many of the foods on the lists share is that a cup is only about 30 calories. Some of them go up as high as 100 calories per cup. Luckily, there are reputable health and diet sites that refute this diet myth.
These foods are great items to eat when you're trying to lose weight. They will fill you up for less of a caloric hit than pasta or most other foods. If you're trying to decide what food to stock for munching or to use for dinner or dessert, please check out the lists that are linked above. They are a great way to help you make sure you are keeping healthy food in your house. Remember, they will not subtract from your total calorie consumption, though.