I think most people are probably familiar with the basic idea of It, in the same way that everyone understands generally how Freddy Krueger or Jason Voorhees operates. Essentially, the town of Derry is home to a fear-eating monster that often disguises itself as a clown.
The story is split over two time periods – one in which a group of outcast children are hunted by the monster, and one years later in which the outcast children, now adults, return to Derry to deal with the monster again. As you might imagine, it’s quite a long book.
The 1990 miniseries was an attempt to take the massive and complex story of It and distill it down to a three-hour running time. It was reasonably successful.