Since the gravitational force is experienced by all matter in the universe, from the largest galaxies down to the smallest particles, it is often called universal gravitation.  Sir Isaac Newton was the first to fully recognize that the force holding any object to the earth is the same as the force holding the moon, the planets, and other heavenly bodies in their orbits.  According to Newton's law of universal gravitation, the force between any two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.  The constant of proportionality in this law is known as the gravitational constant; it is usually represented by the symbol G and has the value 6.672×10^-11 m³/kg·sec² in the meter-kilogram-second (mks) system of units.  Very accurate early measurements of the value of G were made by Henry Cavendish.[1]

$$G\frac{{{M_1} \cdot {M_2}}}{{{L^2}}} = F$$

In the above equation, G is the Newton gravitational constant, M₁ and M₂ are two masses, L is the distance between the masses, and F is the force between the masses.

It is not necessary to elaborate further on the gravitational law since information is widely available concerning its nature.

[1] "Gravitation ," The Columbia Encyclopedia , 6th ed.