The Ray Bradbury Award is given out every year with the Nebula Awards but is not a Nebula Award in itself. Like the Nebula Awards, the final ballot and the eventual winner are decided by votes from members of SFWA, the Science Fiction and Fantasy Writers of America (which despite the name has an international membership).
I like to use the award every year as a sampler of well-loved science fiction and fantasy movies from the previous year. I have been very happy with this tactic, and this year is no exception. I try to watch every movie on the ballot that I can find by rental (usually via RedBox, or occasionally from Comcast On Demand) and review them all within the voting period.