From Wikipedia (because I'm too lazy to summarize):
"After an alien race called the Formics (also known as the "Buggers" in the book) attacks Earth in 2086, the International Fleet prepares for the next invasion by training the best young children to find the future candidate to lead the International Fleet and fill the shoes of the legendary war hero Mazer Rackham. Ender Wiggin (Asa Butterfield) is pulled out of his Earth school to join the International Fleet and attend the legendary Battle School, located in Earth orbit. He progresses rapidly through a series of increasingly complex war games while simultaneously gaining the respect of his peers. As a result, Ender is soon chosen by Colonel Graff (Harrison Ford) to be the military's next Mazer Rackham. The boy progresses to Command School, where he receives training from Mazer Rackham (Ben Kingsley) himself. After a series of grueling battle simulations, Ender leads his peers into a war that will determine the future of Earth and the human race."
I think the movie did a good job of hitting all the major plot points, but it didn't spend enough time developing the characters. Harrison Ford and Viola Davis have too many scenes where they have the same argument over and over - whether Ender is cut out to be the leader he's being trained to be, and whether the training is going to psychologically damage him.
I really liked the zero gravity training room where teams battle each other with laser guns. It's like playing Laser Tag in space. I think kids especially will really enjoy those scenes.
But mostly the movie just felt monotonous to me. He's in class learning battle strategy, he's getting into fights, laser tag scene, Ford and Davis argue, repeat. I just kind of got bored, and I didn't really feel for Ender as a character. Unless you're a big fan of the book, you can skip this movie.