Fat provides us with an abundance of energy, whereas carbohydrates, or glycogen, can only provide us with short bursts of energy. Especially since a diet high in fat has been associated with with so many diseases. So a diet high in fat means a diet high in calories. Anyway you look at it, the more fat you eat, the fatter you will be, keeping in mind that if you overeat ANYof the nutrients you will gain weight. Now, we do need some fat in our diets, for the functions listed above.
Saturated fats are the BAD fats.
Saturated fats are found in foods like meats, whole milk dairy products, and some oils like coconut, palm kernel, and sadly enough the cocoa fat in chocolate. By cutting your fats in total, you will usually cut your saturated fats as well. Unsaturated fats are better, and they make up the remainder of the fat content in your foods. Vegetable foods, like oils, contain a larger proportion of unsaturated fats. There are monounsaturated fats and polyunsaturated fats, depending on how many points of the fat molecule have hydrogen attached.