It is a constant that is the same throughout the universe and never depends on anything. The value of G is approximately 6.67 10-11. This value is obviously very small but it still affects answers. Some scientists believe that the value of G is actually diminishing as time goes by, but very slowly. The gravitational force is one of the four fundamental forces of the universe, along with the electromagnetic, strong nuclear and weak nuclear forces.
Gravity is fairly weak.
Between a proton and electron the force of gravity is 5 10-40 times as strong as the electromagnetic force. However, gravity does have an infinite range. Although Newton's theories were landmark achievements and are usually very accurate, in some situations they fall apart. Einstein came up with the theory of relativity to fix some discrepencies in Newton's work, introducing ideas such as spacetime and gravity waves. The normal person working with normal objects, however, will find that Newton's theories work as accurately as can be measured.