Weight loss diets tend to break down into two basic strategies of getting the weight off: Calorie reduction or hormone manipulation. Calorie reduction is pretty straight-forward: cut back your daily calorie intake in an effort to burn more calories in a day than you're taking in. The idea is that your body will resort to using it's fat stores (the calories stored in your body) for energy.
The latter is a bit more in-depth. Certain foods provoke different chemical and hormonal responses. The one that interests many with fat loss is insulin release. Insulin is a powerful hormone that triggers the body to draw sugar into the muscle cells and tells the body to hold onto fat. Moderating insulin is crucial for weight loss. This is the theory behind low-carb diets. If you moderate the glycemic load into your body, you'll lose weight.
One thing that I've never understood is why some insist that weight loss has got to be one or the other. Maybe it has to do with businesses being set up sell clients on losing weight one way or the other that fuels this competition. Frankly, it comes down to someone's eating habits. It's entirely possible to get fat on 2000 calories a day if someone's eating a high glycemic load on a regular basis. Subsequently, there are some who eat a 2,600 calorie diet consisting of low carb foods who still manage to lose weight.
The bottom line is that weight loss happens for a number of reasons and are more personal than many would have you believe. If you're having trouble losing weight, then you need to write down EVERYTHING YOU EAT AND DRINK. Then, go to a web site like Calorieking.com, check out a few of your day's logs, and take an average of how many calories you're taking in a day. If it isn't very high, then check out the foods that you're eating. If they're a high glycemic index or glycemic load, then you need to change what you're eating.