Aura is usually thought as an indication of radiant light drawn around the head of a saint.
All bodies emit heat radiation or vibrations. With non-living bodies, since there is no immediate change in their structure, this emission remains constant. In living bodies cells are constantly being destroyed and created; the rate of vibrations are continuously changing. This is the superficial or logical explanation of what auras are and why they are constant or changing.
A lot of 'scientific' study of physical auras has been done in the West in recent years. Researchers have variously called it the human atmosphere, force field, and bio-energy. The fundamental substance of aura is best described by the Sanskrit term, prana. The colours of our mental states are manifested in the pranic substance of our auras.
What does your aura tell you? Your aura indicates your state of health to come, specific diseases that are manifested or are going to manifest in your body, your mental and emotional state, your level of intelligence, and how evolved you are as an individual. Assigning meanings to specific colors is of no use to the novice. The positioning, thickness, luminescence, shade and changeability are some of the factors that are taken into account in interpretation. The interpretation is always left to the guru.
(Source: Speaking Tree session of Times of India dated February 6, 2009)