In 1940, the doctor and psychologist, William Sheldon,  introduced three genetically determined 'body types' to the world of human aesthetics.  In "The Varieties of Human Physique" he describes:

   The Ectomorph (our technical term is 'Weed') who is long and thin.  They are what weightlifting aficionados call "hard gainers,", or even  'weaklings', that is  individuals who have a difficult time 'beefing up'. Ectomorphs carry  little fat and while they are typically not gifted with natural muscle, they have  the advantage of not having to worry about gaining too much fat either. 

 The  ectomorph who wishes to broaden out and become more muscular  needs to train intensely, but relatively infrequently to avoid overtraining. Basic  general excursuses are sufficient, best conducted in sessions of no longer than 45  minutes, three times  per week. While aerobic exercise is important for everyone seeking  optimal health, the ectomorph can easily over-train and should moderate exercise in duration,  frequency, and intensity.  Caloric intake must be high and adequate protein, carbohydrate, and essential  fats be included for effective protein synthesis. 

 The Mesomorph is built for athletics with the classical 'V-shape',  that is, wide shoulders,  narrow hips, strong thighs. Mesomorphs easily add muscle and their bodies are far more forgiving of training mistakes. They are natural  athletes, whose  bodies respond well to variations in training, between exercises that  focus on fast twitch muscle fibers (heavier weights) and slower  twitch fibers  (higher repetition ranges).

 The Endomorph tends to be rounder in appearance and has a  softer body due to higher proportions of bodyfat. The "pear shape" would be typical  of an endomorph as there is usually significant accumulation of fat around  the hips and the midsection. Endomorphs attempting to transform their  bodies into lean physiques must be diligent in their meal frequency and must keep their metabolism stoked up while being extremely careful of their sugar  and fat intake. They need to weight train a minimum of three times per week and exercise aerobically at least four or five days weekly. As they  begin to add muscle they will further boost metabolism and speed the rate  of fat loss.

However,  few of us fall cleanly into one category. In fact, Sheldon developed a 7-point scale with Ectomorph being one extreme, Endomorph being the other.