A druid is a type of supernatural who's power is to call upon certain deities. Druids, although mentioned since Stolen, were not actually seen until Everett Weber made a (brief) appearance in Industrial Magic. Although druids may call upon their deity, it is up to the deity to decide whether or not they wish to answer a druid's "call". (As shown in Industrial Magic when Everett tried to call his deity, Esus, for help...but he chose not to intervene in the situation.)
A druids deity does not need to be called upon to intervene, as shown in Industrial Magic when Esus tried to enter the court room to save Everett, but was blocked out by the Cabal shamans.