Magic in the Dresden universe is energy that is pulled from the various sources around a person, the world, and life.
Such power is altering the rules of reailty, but now much it alters relies on the magic around a person.
Various objects, items, totems, and artifacts of belief increase the ability, focus, and strength of a person. Magic can still be done, but either the scale is decreased or it takes energy out of the person casting it.
Magic can be enhanced by outside items such as lighting (Dangerous using it as a lighting rod), blood sacrifice, or objects that have a large amount of belief.