Dinosaurs are a group of reptiles that is
composed of two great subcategories. One is that
the Bird-hipped dinosaurs or the ornithischians and the other is the reptile-hipped dinosaurs, or saurischians. So basically dinosaurs are all the
animals that are descended from the last common
ancestor of those two groups, the saurischians and the ornithischians. An interesting fact is that birds, which are a kind of dinosaur are saurischians so that they're in the
reptile-hipped group of dinosaurs not in the bird-hipped group.