It is a shapechanger and has no set form. When feeding, It takes the form of whatever Its victim fears most. Throughout the novel, It appears most often in the guise of Pennywise the Dancing Clown. Pennywise appears human at first glance, but on closer inspection anomalies (such as abnormal or changing eye color) are apparent.
Its true form on the physical plane is a large spiderlike creature. This is not Its actual appearance, but is instead the closest approximation the human mind can perceive when looking at It in this state. Its real body is the Deadlights, a mass of writhing orange light lying within an otherdimensional space called the Macroverse. Looking at the Deadlights can cause catatonia or even death.
It arrived on Earth in an unknown period of the distant past, at some point between the extinction of dinosaurs and the evolution of humans. It came to Earth from the Macroverse in a spacecraft or meteor and crash-landed in an area that would later become Derry, Maine.
It lay dormant in the region for millions of years until the arrival of colonists from Europe. It awoke and fed on the populace at 27-year intervals. At one point in the early 1700's, It depopulated the entire town of Derry, but It later began to feed in a more discriminating way. It instead began targeting children, while beginning and ending each spree with a large-scale disaster of some kind.