Dinosauria: Systematics

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With the discovery of many new species since the 1840s, the Dinosauria now contains two major groups of dinosaurs: the Ornithischia, or "bird-hipped" dinosaurs, and the Saurischia, or "lizard-hipped" dinosaurs. The division between the two groups was made by H.G. Seeley in 1888. The etymology behind the two names ("bird-hipped" vs. "lizard-hipped") is not very accurate, since some saurischians had bird-like hips, and ornithischians' hips were somewhat birdlike due to convergent evolution, not due to direct ancestry. In fact, birds are saurischians!

Saurischia contains two main groups:

Sauropodomorpha is the first group, and includes both the "prosauropods" — a probably paraphyletic (artificial) group of largely herbivorous, dominantly quadrupedal dinosaurs such as Plateosaurus; and the Sauropoda: long-necked, long-tailed, enormous herbivorous dinosaurs like Apatosaurus (formerly called Brontosaurus), Brachiosaurus, and Diplodocus.
Theropoda is the second saurischian group, consisting of the carnivorous dinosaurs. The Theropoda includes some extinct dinosaurs such as Tyrannosaurus rex and Dilophosaurus, but it also includes the living dinosaurs — the birds.

Ornithischia contains several groups of herbivorous dinosaurs, including several basal groups, but primarily three large ones:

Thyreophora includes the various armored dinosaurs, like Stegosaurus and Ankylosaurus.
Marginocephalia consists mainly of the pachycephalosaurs (bone-heads) and the ceratopsians (horned dinosaurs), like Triceratops.
Ornithopoda is the third Ornithischian group, and includes not only small bipedal plant-eaters like Heterodontosaurus, but also the often large hadrosaurs, or "duck-billed dinosaurs," like Maiasaura and Edmontosaurus.

Visit UCMP's Dinolinks for lists of all kinds of dinosaur-related resources.

Sources:

Weishampel, D.B., P. Dodson, and H. Osmolska (eds.). 1990. The Dinosauria. University of California Press, Berkeley. 733 pp.