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Gravity is a natural phenomenon in the universe due  to which every body having  a mass attracts every other body having a mass.  It is discovered by the Great scientist Isac Newton.

   Gravity = gravitational Force of attraction = G M m / d² ,
               where G = universal gravitational constant.
                     M and m are masses of two objects
                     d = distance between centers of mass of the objects

The force of attraction is called force of gravitation or gravity.  On Earth the force of attraction exerted by the Earth on our body gives rise to our weight.  It is the force with which we drop back to the Surface of Earth, when we jump a few feet high.

       For  an object on surface of Earth,  gravitational Force = weight
                         = G Me m / Re²  = m g,          where g = G Me/Re²
                         Me = mass of Earth,    and    Re = radius of Earth

The acceleration with which we fall back on Earth's surface is called the acceleration due to gravity and is denoted by "g".

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We know that the moon goes around the earth. An object when thrown upwards,reaches a certain height and then falls downwards. It is said that when Newton was sitting under a tree, an apple fell on him. The fall of the apple made Newton start thinking.He thought that: if the earth can attract an apple, can it not attract the moon?

Gravitation is a weak force unless large masses are involved.

Answer referred from NCERT science book class 9th. 
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