The story is set in the future on Earth: Aliens have attacked the planet, and mankind has made a narrow escape. Now the danger is to be eradicated once and for all, and the world government has everything it needs but the right (bright) commander.
Thus the government takes to monitoring little children, to see if they might be the geniuses they're looking for. - We hear of Andrew Wiggin, whom everyone calls "Ender" and who seems to be the brightest of the pack. We observe his training at Battle School, a training which consists mostly of games. But how will he fare in the "final game" against the enemy? Will he break before the showdown, like so many before him?
Large parts of the story are devoted to the social difficulties gifted little Ender has to face because of his outstanding abilities. It seems that because of that, the book has become somewhat of a cult book among those who think they're geniuses - at least that's what I read out of Card's introduction to the revised edition of 1991.
I would not hesitate to recommend this book even to the reader with only average brightness, though :-)... for a Science-Fiction story, there's astonishingly litte technology babble in it, it's intense, not too voluminous and provided me with a lot of thoughts nevertheless.
The book has won the "Hugo" and "Nebula" awards. It's available in many editions, from the collector's leatherbound for $200 down to the mass market paperback. There are numerous websites devoted to "Ender's Game", among them www.ender.com.