Auras are the living energy given off by creatures which is invisible to all except those with Aura Sight. This ability is predominantly used by Lucario, who have the ability naturally, but Togekiss also have the ability to train their Aura Sight. However, to train their Aura Sight, Togekiss need to have the attack Aura Sphere because it puts them more in-touch with Aura.
There is a lot of wild speculation about Aura Sight, mainly by those who don't have it. The Seer, when looking at Auras, simply sees life energy in the shape of a body. Auras also give off a 'feel', which can help differentiate between different people. The Seer can recognize people by their Auras if t he Seer knows them personally and are familiar with the person. Thusly, if a Seer can get the general feel of someone's Aura they could make a semi-informed guess at who the person is but guesses are, obviously, not always accurate. Also, Seers see a person's real Aura so if someone acts differently than they feel, it could throw a Seer off track.
Aura Sight can see no farther than the natural sight range, and even then only if the ability has been honed and trained. Untrained Lucario see a foggy haze outside their Aura Sight range. Togekiss have no natural Aura Sight so when fully trained and utilized one can only see to a distance of around 100 yards, the length of a football field.