The most common body types cited in western culture tend to be those hypothesized by William Herbert Sheldon in his 1954 book Atlas of Men. These types are named after the germ layers of embryonic development, which are called ectoderm (the parts that become the nervous system and skin), endoderm (becomes the digestive tract), and the mesoderm (the parts that become muscle, blood vessels, and the heart). These then developed into the Ectomorph, the Endomorph, and the Mesomorph. There are psychological aspects attributed to each body type, but here I will only go over the physical traits.
The Ectomorph is generally agreed to have an easy time remaining thin. This body type tends to have the long, lean muscles you might see in a swimmer or Pilates instructor. Some might call this body type frail because it tends to have smaller bones and be so delicate. If you are an Ectomorph, you may find that weight training can be too strenuous if done for too long or too often, so be careful and pay attention to how your body feels. It can also be hard for this type to gain weight. Your diet should be high in calories of good quality until you get to a weight at which you are comfortable.
The Endomorph has no trouble at all gaining and storing fat. This body type is associated with the word "big" (as in waist and bone structure). To get into better shape, it is suggested that Endomorphs concentrate on increasing muscle and reducing fat. This improves your ability to burn calories, since just having more muscle on your body will automatically raise your metabolism.
The Mesomorph tends to be stereotyped in cultures as the "hard worker" or "popular" one. If you have this body, you will find that you are naturally fit. This does not mean that you cannot get out of shape and gain unnecessary weight, but you'll find that it's easier to build muscle for you than for the other body types. Do not worry too much about becoming too big. When you reach your preferred size, simply weight train with high repetitions using a reasonably sized weight and you will find that you can remain at your current level of fitness.