Size and Weight:
As you may have guessed from its name, Greek for "parrot lizard," what set Psittacosaurus apart from other dinosaurs of the Cretaceous period was its distinctly un-dinosaur-like head. This plant-eater's curved beak made it somewhat reminiscent of a parrot, but otherwise, its squat noggin was distinctly tortoise-like. (One shouldn't draw too much from this analogy; Psittacosaurus, and other ornithischian dinosaurs like it, weren't directly ancestral to modern birds, an honor that belongs to saurischian dinosaurs.)
Although it's often depicted in a four-legged posture, paleontologists believe some species of Psittacosaurus (there are at least 10 currently named) walked or ran on two legs. (A new study concludes that this dinosaur scuttled around on four legs as a juvenile, then assumed a bipedal posture thanks to a growth spurt in its hind legs.) Psittacosaurus seems to have led a relatively quiet life, although the horns on its face--probably a sexually selected characteristic--indicate that the males may have engaged in combat with each other for the right to mate with females. There's also solid evidence that Psittacosaurus cared for its young after they hatched, like the distantly related duck-billed dinosaurs Maiasaura and Hypacrosaurus.
By the way, you wouldn't know it from its small, unprepossessing appearance (six feet from head to tail and 200 pounds, max, for the largest species), but Psittacosaurus is classified as a ceratopsian--the family of horned, frilled dinosaurs the most famous members of which were the much later Triceratops, Protoceratops, and Styracosaurus. In fact, Psittacosaurus was one of the most "basal" ceratopsians, predated only by the late Jurassic Chaoyangsaurus and itself a close cousin to a bewildering array of proto-ceratopsian genera, including Yinlong and Leptoceratops.