An ad replaces the "screen saver" when you unlock the device (until you swipe it away), and there's a small banner at the bottom of the book list in the main menu. You probably bought a more expensive version with no ads. They can also be removed later for a small fee, or you can simply never enable wi-fi and there will be no ads at all.

There's a specific, cheaper version (10 euros cheaper in France) that has ads in.