The aura is a type of energy field that exudes from an object, living or otherwise. It is interpreted as the effect of one or more things: the energy of the chakras, the result of the interaction of the five elements (Air, Fire, Water, Earth, Spirit) within the body, an emanation of the astral body, etc. It is not a new concept. The main scientific argument against the existence of auras is that scientific equipment, which can detect things far outside the human perception range, can't detect any sort of "energy field" around people, much less anything else. However, the aura is not physical, but astral/spiritual in nature. This is why not everyone can see them, and why scientific equipment can't detect them. It requires training and a certain ability to see them. It is a psychic detection, which is interpreted as a physical sensation (seeing). It can be experienced as being of varied color, brightness, intensity, and size, the overall appearance informing the reader of the spiritual, mental, and physical state of the subject.
Tests have been done to authenticate the abilities of psychics to detect human auras. All of the results indicated that the psychics couldn't see anything at all because they're success rate was random (about the same level of success you'd attain from guessing). Another possible explanation is that what the skeptics didn't understand is that EVERYTHING has an aura - not just people. Anything with an aura that is near to a human will interfere with the ability to see and/or interpret the human's aura. I'm not saying this explains their failure, only that it could be a possibility.
When learning to read auras, keep in mind that not only humans have them - rocks, trees, animals, rivers, etc. all have auras. You can practice on yourself, friends, pets, your garden, etc. Below are links that explain the aura (from different perspectives) and teach you how to read them: