In the novel, It is a shapeshifting monster who usually takes the form of Pennywise the Dancing Clown, originating in a void containing and surrounding the Universe—a place referred to in the novel as the "Macroverse".
It Is Actually An Ancient Cosmic Force Of Destruction
Pennywise is just a mask, an act that It uses when convenient. It is the real monster behind the monster. If you want to get technical, It is an alien, but It's origin goes well beyond that. It is actually an ancient cosmic deity.
Pennywise the Clown is not a clown at all. The Pennywise the clown origin story sees an ancient, evil being that is perhaps as old as the universe itself. In the novel, however, the entity IT/Pennywise has a real name — Bob Gray. Bob Gray isn't human, however, and shares the same origin as IT.
(According to King's book, Pennywise actually is a spider—sort of. That's really just the closest visual approximation to what Pennywise's true form actually is, at least to our puny human minds.)
Stephen King's IT introduced readers to a one-of-a-kind creature that can take any form, the most common one being Pennywise the Dancing Clown, and as menacing as it is, this creature has one big enemy it's truly scared of: Maturin, the turtle.
However, It must surrender the the laws of whatever shape It takes. Pennywise's strength is also his weakness. For example, if he were to take the shape of a werewolf (as he does in the novel), silver bullets would harm him.
Throughout Stephen King's It, Pennywise is referred to as a male, but author Stephen King pulled a fast one on readers in the It book ending by revealing the creature's true form was a pregnant spider, implying that it is, in fact, biologically female.
If we're talking Pennywise per se, it is "male," as shown in the old b&w photographs of Derry, scenes in sewer, etc. But as others have said, Pennywise is merely one manifestation of IT as a larger entity, itself which is not inherently one specific sex or gender but whatever form of sex or gender IT prefers.
The 27-year cycle refers to the extended interval between waking periods of the extra-dimensional shapeshifter in the novel IT, as well as the interval between occurences of The Troubles (outbreaks of increased supernatural activity) in the television series Haven.
The entire town of Derry is essentially infected with ITs evil. That's why children are bullied, parents are abusive, and adults seem not to see the rampant dysfunction around them.
Kersh is Pennywise's daughter. Or at least the daughter of the man who originally had Pennywise's face. Additionally, the trailer offers a first glimpse of Bill (James McAvoy), Ben (Jay Ryan), Richie (Bill Hader), Eddie (James Ransone), and Mike (Isaiah Mustafa) back together.
In their final battle, the Losers' Club was able to kill Pennywise by reducing him to a child-like form as they overcame their fears. In the moments before Pennywise died, his fear is reminiscent of Georgie's own, therefore closing the cyclical nature of Pennywise's murders.
The Deadlights are the true form of Pennywise/IT. The Deadlights are orange writhing lights that exist in Todash Darkness. Pennywise uses the Deadlights to break a person's mind because one look at the Deadlights will make a person go insane due to it not being able to be comprehended by a human mind.
Pennywise is just the form the creature seems to take on the most often. It is actually a malevolent, nigh immortal shapeshifting alien/Demonic god creature very much like a lovcraftian monstrosity.
According to It, when humans got scared, "all the chemicals of fear flooded the body and salted the meat". This is why he prefers to feast on children -- their fears are simple, pure, and powerful compared to the complex, pathological fears of adults. Basically, children are delicious.
Regardless, she's one nasty beast and not alone due to the egg sack she's carrying. Unfortunately, King's imagination ran a little wilder than alluded to in the movies or earlier in the books, and the consequence of that is a pregnant Pennywise.
This twisted extra-dimensional entity had a form a human could only comprehend as a giant spider, but It would almost always disguise Itself as a clown named "Pennywise the Dancing Clown" to attract children so that It could capture and kill them as they were an easier target.
Bill Denbrough described them as writhing, destroying orange lights, and IT as an endless, crawling, hairy creature made of that same light. The theory, then, suggests that the balloons are a representation of the deadlights and contain the souls of the children killed by IT.
The rest of the group realizes they can defeat it by shrinking it and therefore weakening it, which they achieve by standing up to it. They take out IT's heart and proceed to crush it, finally killing it. Eddie dies and his body is left behind.
“I told Bill, 'I want it in both directions, I want you to send your eyes looking away and give a really unsettling look'. He said, 'I can do it', so he did it. What are the chances? It's one in a million and he was cast already.
It's all about imagination. Kids tend to have stronger imaginations than adults, so it's easier to play on their fears. (Note how Bev's father couldn't see the blood that came out of the drain.)
Pennywise does not need to scare his victims in order to kill them, he simply prefers to. The reason he does this is because it supppsedly tastes better. He says it's like "salting the meat." Pennywise feeds off the fear as much as he does the consumption of Children.
Now, when the Losers crushed IT's heart while in its spider form, they killed IT's physical form and severed its connection to Derry, thus why the town was destroyed, but the true entity still lives in another plane of existence.
