Note: This text is dated and represents a view that I used to use but now see the closed nature of it an inflexibility to communicate the multitude of the variety of Shadows. It is an example and you will find many people still adhering and giving similar explanations which may be misleading if you do not understand that there are alternatives. It is still valid, though I may no longer phrase things in the ways stated below I believe it should still be able to provide you with some insight.
The Light and Dark have their definitions but I for one have never found anyone able to give a complete definition. The Grey is supposed to be somewhere in between the others and here is my attempt to explain what the Shadow are. The first paragraph is paraphrased, with alterations from one of original set of lectures at the Force Academy. Without further ado, here is the text.
What are the Shadows?
The Force is traditionally broken down into two unique aspects Dark and Light. Yet the question is often asked, "is the Force really so polarized?". If the Dark and Light sides of the Force are like two sides of one coin then there would be no confusion. However the Force is an energy field, and such cannot be classified in this way. The force in essence is pure energy neither good nor evil, these are sentient concepts and are injected into the force by conduits or wielders of the Force. Therefore we should not consider the Force as Light and Dark sides, for the Force cannot be classified into such opposing categories.
Some people see the Shadows as a group of people who 'sit on the fence' and are indecisive in choosing how they are to act in situations. I would like to state for the record that we are as decisive as needs be, however we will not limit ourselves by the usage of restricting and potentially incorrect labels. Good or bad, right or wrong are very, very dubious. How many times have you started to do something because it seemed so right and so good but regretted it the moment you did it? I'm sure we all have even though some would be less willing to admit to it.
Another way to explain out of the label of 'Fence-sitters' is the simple and unexplained sentence "There is no fence". This is true, this is very true however it is often scoffed at by people. The only explanation I can see for that is through closed-minded individuals who have become so caught up in their polarized ways of thinking that they do not even see anything else as an option.
So how much closer are we to defining the Shadow? Well there are many terms you will hear used such as perceptions, equilibrium/balance, unity and the whole. These will all be explained in other documents but for now I leave you with the following statement.
A Shadow is an object, creature or being that exists in its natural form, It is the truth. A shadow who appears to have Light or Dark tendencies is merely that. APPEARING to have Light or Dark tendencies to another persons perceptions. Perceptions which we as Shadow believe we can do better without.
"...we must follow the truth alone. It is our only guiding star: hoc signo vinces" - Sir James George Fraser, Golden Bough
Peace be with you.