Calories and Metabolism

When people are trying to lose weight, it is common sense that if you consume more calories than you burn you will gain weight. But calorie counting alone isn't going to work for everyone, and won't necessarily help with overall health. The quality of the calories you consume are just as important as how many you consume.
Humans have been evolving for tens of thousands of years. We are very capable of processing things that have been around for generations. This is why fruits and vegetables are the easiest thing to digest. Our bodies can efficiently use the nutrients and eliminate the waste. But in the last 50 or 60 years, food has evolved and changed at a rate much faster than the human body. We aren't as capable of digesting the artificial ingredients and chemical additives that are now considered food. The more the food resembles something your ancestors would have eaten, the better chance you have of properly digesting it. If you don't recognize the ingredients or have no idea how to go about making it yourself, your body will probably be just as confused as how to use it for energy. And if it isn't used, it is stored.
So, if you compare a person eating 2,000 calories a day of fresh fruits and vegetables, whole grains, seeds and nuts with someone eating 2,000 calories a day of fast food, processed food, refined sugars and white flour, chances are that the first person will be healthier and have a better metabolism (assuming other lifestyle factors are similar). Depending on how much weight a person wants to lose, they may need to only change the kinds of foods they're eating without changing the total calories. That being said, a person who needs to lose a significant amount of weight will need to restrict calories along with the kinds of foods they are eating.
As Americans, we have food available to us 24 hours a day. You can get food almost anywhere you go. So instead of eating when our bodies tell us we're hungry, we're eating just because the food is there. It has become a social expectation to have food available. Not to mention that portion sizes have become so grossly over sized. A pasta meal at an Italian chain will serve what is actually 6 servings on one plate. And that is after appetizers, endless bread sticks and free refills of beverages. People want to "get their money's worth" when they are going out to eat, but it's easy to consume an entire day's worth of calories in one meal.
One of the most effective ways to eat a healthier diet is to keep a food journal. For someone who doesn't need to lose weight or doesn't need to lose much, simply writing the food and the amount is easy enough. For a person who needs to lose a lot of weight or really turn their health around, it may be vital to not only record the food and amount, but the calories as well. And depending on the health situation, some people might need to keep track of fat, sugar, sodium or another nutrient. Keeping track of what you eat will give you an idea of how much you're eating (portion control) and also tell you if you're getting a variety of foods. And don't forget to keep track of drinks as well. Eat whole foods, foods in their purest state, unprocessed without any added chemicals or preservatives.

Here are a couple other resources for figuring out calories, serving sizes and recommended intake:
http://nutritiondata.self.com/
http://www.mypyramid.gov/