Body types

Have you ever asked yourself why can't you gain any muscle no matter how much you train, while your friend looks like a greek god after the same amount of working out? Or why can't you get as lean as you'd want to?
It all has to do with the body type you were born with.
Ectomorphs are the thin, low-fat low-muscle guys that find it hard to gain anything.
Endomorphs' body stores fat much more easily, and they have a hard time getting rid of it, but they have a lot of muscle mass too.
Mesomorphs are low in fat, but gaining muscle is not that hard for them as for ectomorphs at all. They are the sport types - often called the "good genetics".

Most people don't have the signs of one type only - almost everyone is a mixture.

Ectomorph 

Small frame and bone structure

Flat chest

Small shoulders

Thin

Lean muscle mass

Finds it hard to gain weight

Fast metabolism

Endomorph 

Soft and round body

Gains muscle and fat very easily

Is generally short and stocky

Round physique

Finds it hard to lose fat

Slow metabolism

Mesomorph 

Athletic

Hard body with well defined muscles

Rectangular shaped body

Strong

Gains muscle easily

Gains fat more easily than ectomorphs