F = GMm/r^2   (gravitational force = G * mass of primary * mass of secondary
                         / orbital radius squared)

  a = GM/r^2    (gravitational acceleration caused by an object = G * mass
                    of that object / orbital radius squared)

When a primary object attracts a secondary object, the force between them
follows an inverse square law with distance, and is proportional to the
product of both masses.  The constant of proportionality, called the
"gravitational constant," is 6.67 * 10^-11  m^3 kg^(-1) s^(-2).
Note that, since F = ma for all systems, the acceleration caused by gravity
can be derived from this formula.

The gravitational force of an object is known as its weight.  This can be 
confusing, because in most of the world we refer to our weight in mass units (for 
example, "I weigh 66 kg; I've lost 2 kg since I started my diet").  In the United 
States, we have the opposite problem: weight is normally given in pounds, which is 
a unit of force, but is commonly used as a unit of mass (for example, "I weigh 145 
pounds; I've lost 5 pounds since I started my diet").  Avoid befuddlement; in your 
mind, keep the vernacular weight straight from the gravitational force weight!