Who is It aka Pennywise aka Robert 'Bob' Gray from It and what is their importance?

It, or Pennywise the Clown, or Robert Gray, is the thing in Derry that kills the children. It has existed in Derry for many years, almost since the beginning of time. Every twenty-five to twenty-seven years, It awakes and feeds on the children of Derry. It likes to feed on children as they have simple fears, and it is easy for It to play on those fears. When It runs into a group of children who do not become frightened by Its tricks, It becomes afraid. It has never known fear, but when It realizes that it can be killed when in Its earthly form, It does become afraid. However, when It survives the first attack by the Losers Club, It believes that they cannot kill It. It thinks that as adults, the children have lost the magic they had when they were young that helped them survive. However, Bill and Richie will prove that It is wrong.