An Aura is a coloured outline emanating from a person. Some believe that the colour of the aura can tell you something about the person. There's no general consensus on what, if anything, each colour means.
Bill Bixby hosted a show in which the Berkeley Psychic Institute's top aura reader had the chance to win $10,000 of James Randi's money. She failed miserably. No aura reader has taken up Randi's million dollar challenge.
ken made a post a while back about seeing colours in music, which can be a result of a condition called synaesthesia. Seeing colours around people is another form of synaesthesia.
Some people believe they can see and 'treat' auras through use of crystals, gems and/or Kirlian photography. You may have seen aura therapy lampooned, on F.R.I.E.N.D.S. for example. Phoebe tries to 'cleanse' Ross' aura by plucking "metaphysical lint" from the air around him.
I think my beliefs about auras are pretty clear. For future reference, I don't believe in anything.