What is the "Golem" in the novel, The Amazing Adventures of Kavalier & Clay?

The Golem is the legendary superhero who defends the Jewish community of Prague, Czechoslovakia, during the Middle Ages. The body of the Golem is hidden in Prague. Bernard Kornblum, with the help of Joe finds the Golem in a large coffin. The Golem is smuggled out of Prague to Lithuania, and in the process Joe is smuggled out of Prague. Much later, the Golem shows up at Sammy's house in Bloomtown.