They are divided into two groups, based on the structure of their hips: the Ornithischians (meaning "Bird-Hipped") and Saurischians (meaning "Lizard-Hipped"). However, do you know what the really big surprise is? Despite what you're probably thinking right now, even though birds are dinosaurs, they are not bird-hipped dinosaurs! Instead, they are actually lizard-hipped dinosaurs.
The similarities in hip structure between ornithischians and modern birds is a classic example of Convergent Evolution, a phenomenon when two or more animals that are not closely related evolve very similar features, in order to adapt to very similar environments. Triceratops and rhinos are another example of Convergent Evolution.