The Capitol shows their power and superiority by having what is called the Hunger Games, which is an event where two people ( a boy and a girl ages 11-18) are chosen from each district to battle to the death in a complicated arena. The arena is huge and tricky, usually a new habitat every year.
When Katniss' little sister is chosen to battle and most probably die, Katniss steps up to take her place. She and the boy (called Peeta Mellark) then go through the complicated first steps of being in the Hunger Games. They are dressed up and put on TV, and everyone in the districts are forced to watch. Then they train to an inch of their lives. Finally, they are put in the arena.
This story has twists and turns you would never expect. I liked it very much. If you want to find out what happens in the arena, or just love to read exciting thrillers, read The Hunger Games, by Suzanne Collins.