"It" apparently originated in a void containing and surrounding the universe, a place referred to in the novel as the "Macroverse" (a concept similar[original research?] to the later established Todash Darkness of The Dark Tower series). It's most commonly used name is Bob Gray or Pennywise (at several points in the novel, It claims its true name to be Robert Gray) and is christened "It" by the group of children who later confront It. Likewise, Its true form is never truly comprehended. Its favorite form is that of a clown (with fangs and large claws when It stalks a child) known as Pennywise the Dancing Clown, and Its final form in the physical realm is that of an enormous female spider, although It is possibly male (or more likely has no gender), the Losers Club considers It female. Its spider form is the closest the human mind can get to approximating Its actual physical form. Its natural form exists in a realm beyond the physical, which It calls the "deadlights." As such, the deadlights are never seen and Its true form outside the physical realm is never revealed, only described as writhing, destructive orange lights. Coming face to face with the deadlights drives any living being instantly insane (a common H. P. Lovecraft device). Bill comes dangerously close to seeing the deadlights, but successfully defeats It before this happens, though during their first confrontation with It, Ben believes that he nearly sees Its true form, and nearly panics as a result. The only known person to face the deadlights and survive is Bill's wife, Audra Phillips, whose encounter with the deadlights nevertheless renders her temporarily catatonic.

Its natural enemy is "The Turtle," another ancient Macroverse dweller resembling a God-like deity, who, eons ago, created our universe, and possibly others. The Turtle shows up again in King's series The Dark Tower. The book suggests that It, along with the Turtle, are themselves creations of a separate, omnipotent creator referred to as "the Other". The Turtle and It are eternal enemies (creation vs. consumption). It arrived in our world in a massive, cataclysmic event similar to an asteroid impact, in the place that would, in time, become Derry, Maine, where It waited for humanity to appear.

Its power is apparently quite vast; during the second Ritual of Chüd, It offers the Losers money, power, and supernatural lifespans if they spare It. Of course, It could merely have been bluffing in order to save Itself. Nonetheless, It is able to manifest in multiple places at once (at one point, It possesses Alvin Marsh, Beverly's father, and Henry Bowers at the same time) and choose to make Itself and anything related to Itself visible to some while invisible to others. When It confronts Richie Tozier in 1985, It threatens to give him prostate cancer, a brain tumor, and turn his tongue into pus, and Richie is convinced that It could actually perform such feats.

Through the novel, some events are described through Its point of view, through which It describes Itself as the "superior" being, with the Turtle as someone "close to his superiority" and humans as mere "toys." It describes that It prefers to kill and devour children, not by nature, but rather because the fears of children are easier to interpret in a physical form and thus children are easier to fill with terror, which It says is akin to "salt(ing) the meat". It is continuously surprised by the children's victories and near the end, It begins to wonder if It perhaps is not as superior as It had once thought. However, It never believes that the individual children are strong enough to defeat It; though It suspects the presence of "the Other" working through them as a group, It dismisses the possibility — an error which proves fatal.

The forms of It
Below are a list of forms taken by the shape-shifting creature:

  • Pennywise the Dancing Clown (Its primary disguise. Mainly used when hunting children. Carries balloons often.)
  • A giant bird, inspired both by a crow that attacked Mike Hanlon as a baby and also Rodan, a giant pteranodon featured in a Japanese horror movie from 1957, when pursuing Mike Hanlon (oddly, It also appears as a giant bird to Will Hanlon, Mike's father, thus making him one of the few adults who can see It)
  • The Werewolf, when It encounters Richie & Bill (wearing a Derry High School blazer inspired by the 1957 horror film, I Was a Teenage Werewolf)
  • The Leper, when Eddie first encounters It under the porch of the house on Neibolt Street
  • A mummy. Ben Hanscom recalls seeing a mummy walking along the frozen canal towards him. It carries balloons that float against the wind.
  • The Eye, when encountering the Losers under the city
  • A swarm of leeches, when attacking Patrick Hockstetter
  • A swarm of piranhas, when Eddie is frightened of crossing the stream
  • The shark from Jaws, seen by a boy named Tommy Vicananza in the Derry canal in 1985
  • Dracula, seen by Ben in the Derry library in 1985. It does not look like any of the traditional variations of Dracula, but rather looks Kurt Barlow from King's own Salem's Lot: very old and with razor blades for teeth. He asks Ben: "What did Stan see before he committed suicide?" The vampire then chops down his mouth and causes his lips to split open and bleed on the floor.
  • A statue of Paul Bunyan attacking Richie Tozier in 1958. In 1985, Richie sees that this statue is replaced by a giant Pennywise.
  • A Doberman pinscher. When It appears to Henry Bowers in 1985 at Juniper Hills Mental Institution, It turns into an 8-foot-tall (2.4 m) dog of this breed because it is the only animal that the guard on duty fears.
  • The moon, while giving Its orders to Its servant, Henry Bowers
  • The witch from "Hansel and Gretel." Beverly Marsh visits her old home to find a woman named Mrs. Kersh living there. Mrs. Kersh then transforms into the witch, showing that she is actually It.
  • Decomposing corpses of children perceived by Stan Uris, as he enters the Standpipe and remembers the tale of the kids who drowned in the water pipe
  • The Deadlights, when Henry Bowers and the Losers encounter It. This is Its true form in the Macroverse. People will see this form of It, if they look too long in the Spider's eyes
  • The Giant Spider, which is Its physical true form on Earth

For millions of years, It dwelt under Derry, awaiting the arrival of humans, which It somehow knew would occur. Once people settled over Its dwelling place, It adopted a cycle of hibernating for long periods and waking approximately every twenty-five to thirty years. Its waking spells are marked by extraordinary violence, which is inexplicably overlooked or outright forgotten by those who witness It. Its awakening and return to hibernation mark the greatest instances of violence during It's time awake.

  • 1715–1716: It awoke.
  • 1740–1743: It awoke and started a three-year reign of terror that culminated with the disappearance of over three hundred settlers from Derry Township, much like the Roanoke Island mystery.
  • 1769–1770: It awoke.
  • 1851: It awoke when a man named John Markson poisoned his family, then committed suicide by eating a white-nightshade mushroom, causing an excruciating death.
  • 1876–1879: It awoke, then went back into hibernation after a group of lumberjacks were found murdered near the Kenduskeag.
  • 1904–1906: It awoke when It rampaged through the woods near Derry, incinerating them. Then It came upon a lumberjack named Claude Heroux, who was hiding in the woods at the time, and, sensing his aggressive nature, possessed him. It, in the body of Claude, murdered a number of men in a bar with an axe. In a possible self-insertion, one of the victims is Eddie King, a possible reference to Stephen King himself, whose middle name is Edwin. In the novel, the unfortunate King is hacked into pieces, an even more gruesome death than his fellow victims of Heroux. Heroux was promptly pursued by a mob of townsfolk and hanged. It returned to hibernation when the Kitchener Ironworks exploded, killing one hundred and eight people, eighty-eight of them children engaged in an Easter egg hunt.
  • 1929–1930: It awoke when a group of Derry citizens gunned down a group of gangsters known as the Bradley Gang. It returned to hibernation when the Maine Legion of White Decency, a Northern counterpart to the Ku Klux Klan, burned down an African-American army nightclub called "The Black Spot." One survivor, Dick Halloran, appeared in King's earlier novel, The Shining.
  • 1957–1958: It awoke during a great storm which flooded part of the city, and murdered George Denbrough. It then met Its match when the Losers forced It to return to an early hibernation when wounded by the young Bill Denbrough in the first Ritual of Chüd.
  • 1984–1985: It awoke when three young homophobic bullies beat up a young gay couple, Adrian Mellon and Don Hagarty, throwing Mellon off a bridge resulting in It killing him, (which echoed real life events in Maine). It was finally destroyed in the second Ritual of Chüd by the adult Bill Denbrough, Richie Tozier, Beverly Marsh, Eddie Kaspbrak and Ben Hanscom.
  • In the intervening periods between each pair of events, a series of child murders occur, which are never solved. The book's surface explanation as to why these murders are never reported on the national news is that location matters to a news story — a series of murders, no matter how gruesome, doesn't get reported if they happen in a small town. However, the book's implied reason for why the atrocities go unnoticed is far more sinister: It (the monster) won't allow them to be. In fact, Its power over the town is so absolute that Its death in the second Ritual of Chüd causes an enormous storm that damages the downtown part of Derry.

Although It is defeated once and for all by the novel's end, there are hints in King's later works, most notably The Tommyknockers and Dreamcatcher that It is still alive. Furthermore, It is revealed to have laid eggs while in Its spider form shortly before Its defeat. Whether or not every egg has been destroyed never has been and never will be completely resolved. -Wikipedia