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.
Small frame and bone structure
Lean muscle mass
Finds it hard to gain weight
Soft and round body
Gains muscle and fat very easily
Is generally short and stocky
Finds it hard to lose fat
Hard body with well defined muscles
Rectangular shaped body
Gains muscle easily
Gains fat more easily than ectomorphs