Pennywise the Dancing Clown is the favored form of a greater entity, IT, the namesake of Steven King's novel. IT is a being of unknown age, though in the novel, the creature claims to be as old as time itself. Not only does IT lack a definite age, but it lacks a gender as well. Though the creature is referred to as male for the majority of the novel, IT does take the form of a female spider repeatedly. IT is, as best as can be assumed, a type of light and shadow being, made of chaotic orange light that exists in what the book refers to as a Macroverse. This place holds our own universe as well as the one beyond, where powerful entities such as IT originate. The novel never says conclusively if the orange light being is IT's true form, but it drives viewers to insanity and thus can only be assumed to be the monster's true form.
IT uses the form of Pennywise when on Earth in order to get close enough to lure children. IT prefers his meals to be filled with fright (for it's like salting the meat), and children are especially susceptible to their fear. Thus, Pennywise was created to satisfy specific cravings that the beast has. Every twenty-seven to thirty years, when a crime of viciousness and hate occurs, IT is summoned to Earth to feed. Until an act of equal horror as the first happens, IT continues its reign of terror as Pennywise the Dancing Clown.