Generally, the human body can be broken up into 3 general groups for categorization. These categorizations are Ectomorph, Mesomorph and Endomorph. Having different body types results in 2 main things; diet and exercise. Let us first touch on Ectomorphs.
Ectomorph are generally lean and skinny by nature. This characteristic usually causes the individual to face difficulty putting on weight no matter how much they consume. They are also usually framed by thin limbs, narrow shoulders and lower body fat percentage. In the current age of fashion, this may benefit the ladies better but not so much in a guy wanting to gain that much needed weight to look physically more attractive. The reason behind an ectomorph small frame is a fast metabolism rate. This results in 2 things; a short time for the body to break down and use the energy from the food leaving little to no fat being left behind to store and also the body natural way to turn towards breaking down muscle as a source of energy. This group are usually plagued by significantly less result in terms of weights and resistance training because the training would cause the metabolism to spike even higher and without the proper diet and consistency, this will result in slimmer chance in gaining the much needed mass.
Mesomorph on the other hand are what most men would love to get naturally. Their ability to gain and lose weight just by changing their diet and exercise with a slight variation result in many becoming models and contact sports like rugby or weightlifting. Similarly, the reason behind this ability lies in a normal metabolism rate which burns less readily than the ectomorph and thus do not really need to turn to muscle breakdown as a way to get energy to fuel the body. This group of people can usually keep the muscular results from the gym on for a significantly longer time if they abide by a generally healthy diet.
Endomorphs are on the heavier side of the scale. They are the individuals which find it hard to lose weight and every little snacking and food consumption could result in disastrous result on their body. Similarly to the previous 2 groups, the reason behind endomorphs peculiar structure is because of their slow metabolism rate. This result in more energy being stored for future use rather than being used right now to fuel the body. Endomorphs typically have a higher count of body fat and are usually identified with a wider waistline and flabbier outlook.