It was about the dead coming back to life and devouring the living. It followed a small group of humans trying to survive the dead by hiding out in a house until morning. He followed it up with Dawn of the Dead, which was a allegory for commercialism and America's love of malls, and Day of the Dead, which was about the government's reaction to the zombies and how they studied them. Officially, there's also Land of the Dead, Survival of the Dead, Diary of the Dead and remakes of the earlier films as well.
Since the original title of Night of the Living Dead was Night of the Creeps, the studio changed it after it was made, Romero never retained rights to the title. This caused a myriad of knockoffs under the living dead name, but those above are the only ones created by the master George Romero.
Zombies have become the world's preferred monster with zombie walks, movies, television shows, T-shirts and books. There are people that have made a very good living off zombies and the zombie apocalypse, but Romero was the progenitor of it all. Had it not been for Night of the Living Dead, there would be no World War Z or The Walking Dead.
It's almost unanimously agreed that Romero is and always will be, the king of the living dead.