Theurgy is the act of producing miracles and unbelievable effects. Basically, it is the world's magic.
Ones who can produce such effects are known as Theurges. These Theurges have formed a world-spanning organisation known as the International Guild of Theurges (or IGT for short).
The IGT has divided levels of Theurgy into 9 Circles. Which Circle a Theurge is in generally shows how powerful a Theurge is.