Gravity is the force of attraction between any two bodies which have mass. For example: There is a force of attraction between you and anybody nearby you which have mass. This force is so small that you do not feel it. But the force of attraction between you and earth is huge because the mass of the earth is huge. Earth attracts nearby objects towards its center by the gravitational force of attraction. The gravitational force of attraction by earth on your body is same as your weight on earth.
Laws of gravity was given by Newton:
Newton's law of universal gravitation states that any two bodies in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
G is the gravitational constant and its value is fixed.