Newton's law of universal gravitation states that every point mass attracts every single other point mass by a force pointing along the line intersecting both points. The force is proportional to the product of the two masses and inversely proportional to the square of the distance between them.

$$F$$ = the force between the masses $$G$$ = the gravitational constant $$m1$$= the first mass $$m2$$ = the second mass $$r$$ = distance between the centres of the masses.

More info can be found on Wikipedia: http://en.wikipedia.org/wiki/Newton'slawofuniversalgravitation