It (Stephen King)

"I'LL KILL YOU ALL! Hahaha! I'll Drive you crazy, and I'll kill you all! I'm every nightmare you've ever had, I am your worst dream come true. I'm everything you ever were afraid of...AAARRRHH!!"

- Pennywise the Dancing Clown aka IT. "They float, Georgie. They float... And when you're down here with me, you'll float too!"

IT, also simply known as Pennywise the Dancing Clown, was the main villain of the book and movies of the same name. He was an evil and demonic entity

Film
It was part of the disasters that occur every 30 years in the Maine town of Derry.

Unlike many horror monsters before him, Pennywise the Dancing Clown was not against killing children; in fact, the scary monster actually preferred to kill children as they were an easier target, and perhaps this was why he chose the form of a clown, a figure that both entertains and terrifies many young children. The clown also had razor sharp teeth.

At any rate, as the film progressed, a group of children known ironically as the 'Losers' Club' banded together to kill Pennywise and end his murderous reign--after he had killed the brother of one of them--and apparently succeeded after tracking him down to his lair--however, Pennywise, being a lesser-aspect of a higher being, was not going to stay dead forever, and he swore revenge on the gang for his defeat before he vanished into nothingness.

Years later, Pennywise kept his promise and came after members of the gang, who were all adults in the present day, to kill them. This prompted the gang to reform and battle Pennywise again in order to kill him yet again and save themselves from his wrath -- at the end of the movie, the gang did manage to defeat Pennywise again, but in that final battle, he took the form of a spider-like monster rather than the clown disguise (which he used for most of the film). He was killed when Bill pulled out his heart.

Novel
The film lacks many elements of It that the novel had included. In the novel, It was an eternal entity that was almost as old as time itself. It was the natural enemy of Maturin (The Turtle), who both existed in the Macroverse.

After arriving to Earth, It would sleep for approximently 28 years at a time, then awaken to reek chaos and feed (primarily on the fear of children). It was able to take many mroe forms then the film depicted, including werewolves, bats, leeches, and even Jaws. Anything a child was afraid, It could become.

Also in the novel, It was only able to be stopped when Bill performed the Ritual of Chud.