There are three body types - ectomorphic, endomorphic and mesomorphic - and it's important to understand what body type you have and what the characteristics of that body type are.
Ectomorphic - Ectomorphs have slender frames and are naturally lean. They tend to have muscles that are long and thin, especially in the arms and legs. Ectomorphs have high metabolisms, and they can often eat whatever they want and stay lean. They also can often stay lean while doing little or no exercise. The downside is that ectomorphs usually have a very difficult time gaining muscle.
Endomorphic - Endomorphs have stocky frames and naturally high levels of body fat. They have soft, round bodies with wide waists and hips. Endomorphs have slow metabolisms, and they tend to gain body fat very quickly if they eat too much food or the wrong types of foods. Endomorphs usually have a difficult time losing body fat with diet alone; they need the boost in metabolism that exercise provides.
Mesomorphic - Mesomorphs are naturally lean, muscular and strong. They have small waists and broad shoulders. Mesomorphs gain muscle and strength easily and lose body fat easily. They respond well to almost any type of nutrition and training program.
While most people tend to gravitate towards one body type predominantly, very few people are 100% of one body type. There's usually a mix of two or even all three body types. And while we all have a predominant body type that's genetically influenced, our body type can change as a result of training, nutrition and lifestyle modification.
The key to making changes to your body type, such as losing body fat, gaining muscle or developing a more toned, athletic look, is to understand your body type and then follow the right nutrition and training strategies for your body type.
Using myself as an example, my predominant body type is ectomorphic. But I've been able to develop a more mesomorphic body type by following a good weight training and nutrition program. I would return back to my more natural ectomorphic body type if I stopped following my current weight training and nutrition program.
If you're looking for a good resource that covers training, nutrition and lifestyle strategies for the three body types, check out Burn The Fat, Feed The Muscle. While it's an especially good resource for endomorphs who want to lose body fat and get fitter and healthier, it also contains plenty of information that ectomorphs and mesomorphs will find useful. I used Burn The Fat, Feed The Muscle to help me develop my current weight training and nutrition program.