The Auditors of Reality are the watchers of time and space. It is they who make sure that atoms spin and gravity pulls. If you travel faster than light, it is likely that it is they that will give you the speeding ticket.
The Auditors appear as nothing more than plumes of smoke within grey cloaks hovering in mid-air. Auditors refer to themselves as "Us" or "We" and never "I" or "Me". The Auditors believe that to have a personality is to live and to a being that is as old as time itself, to live is to die, so the moment any Auditor shows a form of emotion or personality, it immediately evaporates and is replaced.
Auditors always work in groups of three so that one can be watched by the other two.
In Reaper Man, the Auditors fired the Death of the Discworld for having too much of a personality, but they let him keep the horse.
In Hogfather, an Auditor speaks with Assassin Guild President Lord Downey, requiring his service to take down "the fat man" to dispel world belief in the Hogfather by Hogswatch day.
In Thief of Time, the Auditors sought to stop time all together. They took on Human forms and ordered a clock to be constructed that would capture Time herself. This would allow them to catch up with their paperwork.