Jack the Giant Slayer was originally going to be called Jack the Giant Killer:

But the new name certainly gives it that lighter, Buffy-like feel. It also distinguishes it from the 1962 movie:

It’s an adaptation of Jack and the Beanstalk:

Jack the Giant Slayer tells the story of an ancient war that is reignited when a young farmhand unwittinglyopens a gateway between our world and a fearsome race of giants. Unleashed on the Earth for the first time in centuries, the giants strive to reclaim the land they once lost, forcing the young man, Jack, into the battle of his life to stop them. Fighting for a kingdom, its people, and the love of a brave princess, he comes face to face with the unstoppable warriors he thought only existed in legend–and gets the chance to become a legend himself.

For the true purist, however, there is only ever the 1917 silent movie, Jack and the Beanstalk: