Whole grains are simply those grain foods that have not undergone any processing, meaning they have all parts of a grain; the endosperm, germ, and bran. Processed, or refined grains have only the endosperm. The endosperm provides nourishment to the germ and is rich in carbohydrates. The germ is the actual embryo of the seed and contains several trace minerals and essential fatty acids, while the bran is the outer layer of the seed and is a rich source of fiber.
Whole-wheat flour, whole wheat bread, brown rice, oatmeal, and even popcorn are whole grain foods. Pasta, white bread, and white rice are just some examples of processed grain foods.
So why are whole grains good for weight loss? Simply, whole grains, containing all parts of the grain, have and abundance of nutrients when compared to processed grains; for instance whole grains have anywhere from 3 to 4 times the amount of dietary fiber. The fiber helps slow down the absorption of carbohydrates into the blood stream - but still supply the body with needed carbs. Processed grains can spike insulin levels and increase body fat. The germ is loaded with nutrients making them valuable for building muscle and metabolizing fat. All three parts combined make for very healthy eating will aid in any weight loss efforts.
When shopping for whole wheat bread, it is important to read the labels. Many companies will add useless substances to the whole wheat bread, significantly diluting its nutritional value. A particular brand of whole wheat bread I ate for months ended up containing high fructose corn syrup, which kind of defeats the purpose. Now I buy my whole wheat from organic grocery stores.