An award made each year by the Science Fiction and Fantasy Writers of America (SFWA), for the best science fiction stories of the previous year. The awards are chosen by a ballot of SFWA members. There are four different story length classes for which stories are nominated and voted: novel, novella, novelette and short story. There is no cash prize associated with the award, the award itself being a transparent block with an embedded glitter spiral nebula; however, publishers never fail to mention that a novel won the award, and as a mark of acclaim by other science fiction writers, it seems reasonable that the Nebula, like the Hugo award, leads to increased sales.
It frequently happens (but not always) that both the writers' award (Nebula) and the fans' award (Hugo) to go to the same stories.