The Dinosauria is an enormous clade of organisms, encompassing 1500 known species and still thousands more undiscovered. The system of species division of the Dinosauria was created by a paleontologist by the name of Harry G. Seeley, whom you may remember from Agrosaurus. Seeley decided that Dinosauria should be split into two main orders, ornithischia, meaning bird hipped, and saurichia, meaning lizard hipped. These names turned out to be rather confusing later on when it was discovered that in reality, saurichians were the ancestors of today's birds. The saurichian group was incredibly diverse, including everything from Tyrannosaurus, to Apatosaurus, to Velociraptor. On the ornithischian side, all animals were herbivores, and dinosaurs like Torosaurus, Stegosaurus, and Parasaurolophus were in this group. Both of these groups then split into families, the most well known on the Saurichian side being the sauropods and theropods, and on the best known on the Ornithaschian side being the ceratopsians, ornithopods, and thyreophorans. The sauropods included pretty much everything with a long neck, like Brachiosaurus. The theropods included the well-known Velociraptor, Tyrannosaurus, and many other bird-like dinosaurs such as Aurornis. The ceratopsians included animals like Triceratops, the ornithopod group was very diverse and included animals like Parasaurolophus and Iguanodon. The thyreophoran group was home to the armored dinosaurs, like Ankylosaurus, and Stegosuaurus. I made up this kind of lame game thing to help explain how all these groups are related, so check it out.
Palmer, Douglas. Dinosaurs. New York: HarperCollins, 2006. Print