The body needs both calories and nutrients to function properly. Unfortunately, not all foods provide quality nutrients. So-called 'junk foods' (e.g., candy bars, sodas, fast-food hamburgers, etc.) are high in calories and in refined sugars and/or saturated fats, but do not provide other worthwhile nutrients. While junk foods can be sources of quick energy, they are bad for health overall. If empty calories replace more important nutrient-dense foods such as fruits, vegetables and low-fat dairy products, the end result is a poorer quality diet. A steady diet of junk food can actually contribute to malnutrition and disease. In children and adolescents a steady diet of junk food might negatively effect growth and development.
In the last century, food producers have taken to refining foods so that they will last longer on supermarket shelves. The process of refining food, however, often damages its nutritional value. The white flour used to make white bread provides a good example. Wheat berries that get ground to make flour have several parts - the bran, or outer protective layer, the germ, or living seed, and the endosperm, which is the starchy food source the germ will consume while growing. White flour is made by removing the germ (which contains oil that would go rancid) and bran parts (which contain fiber) of the wheat berry and grinding what is left. As much of the nutritional value of wheat is found in the germ and bran parts of the wheat berry, white flour ends up having very little nutritional value. Whole wheat bread made from whole wheat flour (so-called because it is made by grinding the whole of the wheat berry including the bran and germ) contains far more nutritional value than white bread.
The best, most nutritional foods to eat tend to be 'whole' foods; minimally processed and arriving at the supermarket with much the same composition they had while growing. Whole grains such as whole wheat, whole oats and beans which retain their beneficial fiber provide more nutrients than highly processed foods and may also reduce the risk for cardiovascular disease and certain cancers. Minimally processed foods containing fiber also help moderate blood glucose levels, aid elimination, assist in lowering blood cholesterol, and make weight management easier.
Organic foods vs. Conventional foods.
Most of the foods and meats found in a regular supermarket are produced on factory farms with the aid of chemical pesticides, hormones and antibiotics. Though most of these chemicals wash away, trace levels remain in foods and get eaten by people. This is of particular concern with regard to meat animals which get raised on pesticide dosed vegetation and concentrate pesticides in their tissues. Many people feel that having such chemicals in their foods is a bad idea that can lead to health problems. The alternative to conventional foods are organic foods, which are often produced on smaller scale 'family' farms and always without using pesticides, hormones or antibiotics. Though they generally cost more, organic foods are less likely to contribute to disease, and more likely to be nutritious than their conventional counterparts. Some people claim organic foods taste better too, but this may be a matter of opinion.
The difference between nutritional food choices and being on a diet
From both nutritional and weight-loss standpoints, it is smarter to eat a balanced nutritional diet all of the time than to eat as one likes (without regard for nutrition) most of the time only to go on a special reducing diet when one feels overweight. The latter option is unsustainable, and sometimes even unhealthy. Though reducing diets work when properly followed, they are not necessarily well balanced nutritionally. Reducing diets also have a fatal flaw which is that they always end. Returning to a poorly-balanced regular diet after going 'off' a reducing diet will all but certainly result in new weight gain.