I haven’t read the book, and I haven’t seen the movie in a while. From what I can recall and what I am gathering online IT is some kind of life form that exists in multiple dimensions or the multiverse that feeds both on human flesh and feeds on negative emotions like fear. Supposedly IT took on the shape of a clown to attract children (the desired prey).
But I’ve heard the book has several parts told from IT’s POV, so maybe there is more to it than that. Does anyone else know?
Nope that is about the long and short of IT 
IT was stranded on earth after falling through one of the multiverse gates that are common in King’s body of work, it seemed rather angry at being stuck here. I think its native form is a giant spider or something, IT came from a technological species with “space” ships. Supposedly IT’s species appears in The Dome as a cameo.
It was basically a Lovecraftian entity of unfathomable shape and form that the human mind, faced with its utter alienness, interpreted as a spider. You know, one of those.
The book, incidentally, is probably the most Stephen Kingy of all of Stephen King’s works.
That’s correct; it is from another dimension. Its true form was described as a writhing, many-legged monstrosity of enormous size with an orange hue. And to see its true form apparently renders people insane. It can take on any form in our dimension, including a clown and spider as well as people its victims know well and trust.
Deadlights.
And for all that, still highly vulnerable to common asthma medication.
Unspeakable Lovecraftian horror is more correct than “spider” - IT changed into whatever the victim feared most*: Richie had his Eye, Stan had the flying bloodsuckers, and all kids in Derry, not just the pro/antagonists, saw the literal manifestation of their fear of adult indifference.
IT was a spider at the end solely because something about IT’s true shape made Bill yell out “it’s a spider!” It was only then the other kids “knew” what they were looking at. But it could just as easily have been a snake or a dinosaur. Or, hell, Bill’s boy-rapin’ uncle for all we know.
To belief. The kids thought the asthma medication was just like battery acid, so they *believed *it was battery acid. IT had been working off the fears (and therefore, beliefs) of children so long, it had made itself susceptible to them. So when kids believed, it came true.
It should be pointed out that the “medication” was a placebo, “Hydrox.” “It’s just water, kid.”