The aura is an energy field that surrounds our bodies - it's a part of our life force.  You can think of it as an "atmosphere" that surrounds us, just like the Earth has an atmosphere, yet it comes from within us.  Every living thing has an energy field or aura surrounding it.  An aura can be seen or sensed by others who are sensitive enough to pick up on it.

What Does this Have to do with Ghosts?

One working theory is that our auras are an extension of our souls, that is, that the aura that surrounds our body originates from our soul and extends to the outside of our bodies.  This can be compared to heat that comes off of a stove or fire.  The stove or fire creates heat, which radiates away from the oven or fire, much like the aura radiates away from the soul.  Auras can be seen in various colors and can usually be affected by mood or health.  The color or strength of an aura can indicate illness. 
When we die, our souls leave our bodies, and with it, the energy of our body which creates the aura.  What happens to the aura?  Since it is just an extension of the soul, the aura simply recedes back into the soul at death.  Souls go on to become either ghosts or spirits, and in either case, the aura literally becomes a part of the ghost or spirit.  Some psychically gifted people claim to be able to see or sense ghosts as having an aura, although an aura is not necessarily a separate energy source that the ghost or spirit creates.

The Discovery of the Aura

In the early 1900's, a Russian technician named Semyon D. Kirlian was repairing equipment in a hospital when he noticed something unusual.  He found that through interaction with electrical currents and photographic plates, he was able to create imprints of living organisms on film.  Semyon, along with his wife, developed the process in 1939, and called it "The Kirlian Effect" (pronounced "Keer-lee-an")  

Kirlian photography involves the transferring of a high frequency charge through a metal plate attached to a Polariod film camera base.  The fingertips are lightly placed on the film which is encased in a light-tight bag, and an electric exposure is made.  Sixty seconds later, after the film develops, the Kirlian photograph is complete, and the subject's energy field is revealed.

With this kind of technology, one can photograph the auras of living things.  Human beings, plants, animals, etc., all create auras.  Auras can vary in color, apparently depending on the subject's mood, level of psychic sensitivity, or health status.  Disease shows up in a disrupted pattern in the aura long before the disease manifests itself in any physically diagnosable form.  Hypnosis, drugs and alcohol can also alter an aura.  And medically speaking, people with certain medical conditions, such as epilepsy, may experience seeing auras just before a seizure event. 

 Interestingly enough, it's been discovered that an aura apparently can't be damaged.  When a part of a leaf is cut off, the aural energy still forms as though the leaf were still complete. 

In a Kirlian photo of a leaf, the developed print shows an aura of light around the leaf.  However, if a portion of the leaf is torn away, and the entire leaf photographed again, the missing portion will oftentimes still appear!  This is known as the "phantom phenomenon", and is a controversial issue.  It may be proof that an "energy body" exists even after the physical body is destroyed, hence possible proof of the existence of ghosts.  This might also explain why most people can still "feel" a limb after an amputation.  Amputees have also reported feeling pain in a limb that is no longer there.  This pain is referred to as "phantom pain" and may have to do with the possible sensory effects of the aura. 

Skeptics who deny the phenomena say it is the product of incompetent photographers.