The first trailer for Attack on Titan shows a frustratingly short glimpse of the human-eating titans, but at least we get a sense of the story's set-up.
One hundred years ago, titans appeared on Earth and gobbled up most of civilization. Those that remained built a massive wall to defend themselves, but the wall sort of sucked. When the giants found the humans, the proceeded to feast again.
The movie is about humanity's fight to not become snacks.