The gravitational constant (G) is an experimentally calculated value that is involved in determining the gravitational force between two objects. Its primary use is outlined in the following equation:
The gravitational constant is therefore a proportionality constant between the gravitational force and the result of the product of the masses of two objects divided by the square of the distance between them. This is an example of an inverse square law. Historically, this constant was first measured using a torsional balance as seen in figure 1.
For more about how the gravitational constant was found please see Physics Central or this Scientific American article on the Puzzling measurement of the Big - G gravitational constant.
The University of Colorado has graciously allowed us to use the following Phet simulation. To get a physical intuition about how the law of gravity works, please explore the simulation below. Notice that even the biggest gravitational force below is still quite small compared to how much a person weighs (about 500-1000 N).