The character is an ancient, trans-dimensional evil entity who preys upon the children (and sometimes adults) of Derry, Maine, roughly every 27 years, using a variety of powers that include the ability to shapeshift, manipulate reality, and go unnoticed by adults.
A god of destruction, not just of our world, but of multiple worlds and even multiple universes. It originated in a vast cosmic void between worlds that has been called, at times, the Macroverse, and in King's Dark Tower series, the Todash Darkness, in a space It refers to only as the Deadlights.
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.
He took the form of a clown most frequently, Mr. Bob Gray or Pennywise, but his true form is an ancient eldritch entity from another universe who landed in the town that would become Derry by way of an asteroid and first awoke in 1715.
That is, when Pennywise morphs into a werewolf, he is subject to the same weaknesses as a werewolf, including silver slugs. In both the original novel and made-for-television adaptation, Pennywise is weakened using silver bullets melted down from earrings.
Down in the sewers, during their final encounter, it's much of the same. Mike brings the gun he uses to kill livestock, and the rest of the Losers come prepared with more rods. Beverly deals the final blow by putting one straight down It's throat.
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.
Muschietti appears to be implying Pennywise was a human familiar who It corrupted and then enjoyed so much It incorporated his shape into its file cabinet of monsters.
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.
You've also probably read all of King's horror novels, including The Dark Tower series, which reveals that Pennywise's archenemy is The Turtle.
Throughout the book, It is generally referred to as male, usually appearing as Pennywise. The Losers come to believe It may be female after seeing it in the form of a monstrous giant spider that lays eggs.
In the history of Derry he kills a lot of people that aren't afraid of him. Sometimes they don't even know he's there. So he can kill people regardless of whether they're afraid of him, he just only eats people that are afraid of him.
The turtle is a force of benevolence and is guided by compassion, whereas IT represents chaos, evil, and fear. They are diametrically opposed. Because they are both interdimensional entities with equal powers, the turtle could kill IT if it wanted to, therefore it's understandable that IT is terrified of it.
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.
As well as having his right arm bitten off, Georgie is trying to crawl away but he was dragged into the sewers and devoured by Pennywise, with an unnamed older woman and her cat being the sole witnesses of the horrific scene, including before Georgie lost his arm.
This is one of the most interesting Hollywood couplings we learned about in quite some time. Apparently, Pennywise and the Babadook are dating. In fact, they're in a long term committed relationship.
Stan Uris (Wyatt Oleff & Andy Bean)
The first member of the Losers' Club to meet their demise, Stan is driven to suicide by his memories of facing off with Pennywise as a child. At least that's how it's explained in the book.
Kersh is Pennywise's daughter.
It's primary goal is to feed on humans, generally preferring children over adults since they were easier to scare and manipulate. According to the creature, frightened flesh tastes better and uses fear to "salt the meat".
Throughout the summer and their trials against Pennywise and the school bullies, the Losers realize that none of the adults in Derry can actually see Pennywise or the horrors he is committing against the children of Derry.
There is nothing that can kill Pennywise. The only weapon is believing, which is a weapon that actually Pennywise uses to kill his victims.
First we have to clarify that Pennywise doesn't kill, he is basically an illusion, It kills. It kills children because they are safer to kill then adults. Children are relatively powerless and most people (outside of friends and family) soon forget about them.
Pennywise's Kills Add Up Over Time
The number is then multiplied by the number of times Pennywise has awakened -- once every 27 years throughout the 270-year history of Derry, equaling ten times -- to come up with a final tally of between 12,000 and 18,000 dead.