In the film: We get to see the thing in all of its alien glory, and then the base is destroyed in the hopes of killing it. We’re left with two survivors, MacReady and Childs, neither trusting the other to be human, slowly freezing to death in the snow.

In the book: The novella ends after the aliens are revealed and their plan of escape is discovered moments before they get away, resulting in a victory for humanity. It’s a more decisive ending.