In rough words, an aura is the energy that the body emits. It is divided in several layers and the easiest to see is the one that corresponds to the etheric body. It can be seen around just about everything, animals, plants, people, things... everything emits energy.... I see the aura of a stone I use as paperweight on my table.
If you read books like Barbara Brennan's "Hands of Light" or Cindy Dale's "New Chakra Healing", there are wonderful explanations of the aura. Also you can spot illnesses in the aura before they affect the physical body, The physical body is in reality the last one to be affected.
There is a free Robert Bruce ebook, "Training to See Auras", which I just read and found very useful (the blue book experiment comes from there) at this page, which I think I already posted in another message
http://www.xehupatl.com/freebooks.htm
Also, it is from one of the layers of the aura that you can access the Akashic Records, which is where our previous life records are archived. These records can be accessed through reiki or energy healing procedures.
There is a small booklet by Ted Andrews which has useful exercises to improve and clear your own aura.