1. Dinosaurs weren't the first reptiles to rule the earth.
The first dinosaurs evolved in the middle to late Triassic period, about 230 million years ago. Before then, the dominant land reptiles were archosaurs ("ruling lizards") and therapsids ("mammal-like reptiles"), and for 20 million or so years after the first dinosaurs appeared the most fearsome reptiles were crocodiles. It was only at the beginning of the Jurassic period, about 200 million years ago, that dinosaurs began their rise to dominance.
2. There are plenty of dinosaurs that haven't been discovered yet.
The only dinosaurs we can know about are the ones that leave fossil remains, but fossilization is an extremely rare process. Fossil evidence is abundant for late Jurassic (150 million years ago) and late Cretaceous (80-65 million years ago) dinosaurs, but long stretches of geologic time, across various continents, remain unaccounted for. One shouldn't take this too far, though: it would be surprising if paleontologists discovered an entirely new and unclassifiable type of dinosaur, since most dinosaur families have been well sorted out.
3. Dinosaurs prospered for over 150 million years.
With our 100-year-max life spans, human beings aren't well adapted to understanding "deep time," as geologists call it. Modern humans have only existed for a few hundred thousand years, and human civilization only got going about 10,000 years ago, mere blinks of the eye by Jurassic time scales. Everyone talks about how dramatically (and irrevocably) they went extinct, but judging by the immense amount of time they lasted, dinosaurs may have been the most successful creatures ever to colonize the earth.
4. The dinosaur kingdom is split into two main groups.
You'd think it would be logical to divide dinosaurs into herbivores (plant eaters) and carnivores (meat eaters), but paleontologists see things differently, distinguishing between saurischian ("lizard-hipped") and ornithischian ("bird-hipped") dinosaurs. Saurischian dinosaurs include carnivorous theropods and herbivorous sauropods, while ornithischians account for the remainder of plant eaters, including hadrosaurs, ornithopods and ceratopsians, among others. Oddly enough, birds evolved from "lizard-hipped," rather than "bird-hipped," dinosaurs!
5. Dinosaurs (almost certainly) evolved into birds.
Not every paleontologist is convinced, and there are some alternate (albeit not widely accepted) theories. But the bulk of the evidence points to modern birds having evolved from small, feathered, theropod dinosaurs during the late Jurassic and Cretaceous periods. Bear in mind, though, that this evolutionary process may have happened more than once, and that there were definitely some "dead ends" along the way (witness the feathered, four-winged Microraptor, which has left no living descendants).
6. Some dinosaurs may have been warm-blooded.
Modern reptiles like turtles and crocodiles are cold-blooded, or "ectothermic," meaning they need to rely on the environment to maintain their internal body temperatures--while modern mammals are warm-blooded, or "endothermic," with active, heat-producing metabolisms that maintain a constant internal body temperature no matter the outside conditions. There's a good case to be made that at least some carnivorous dinosaurs must have been endothermic, since it's hard to imagine an active, predatory lifestyle being fueled by a cold-blooded metabolism.
7. Most dinosaurs were vegetarians.
Giant predators like Tyrannosaurus Rex and Giganotosaurus get all the press, but it's a fact of nature that the meat-eating "apex predators" of any given ecosystem are tiny in number compared to the plant-eating animals they prey on (which can rely on the vast amounts of vegetation needed to sustain such large populations). By analogy with modern habitats in Africa and Asia, herbivorous hadrosaurs, ornithopods and (to a lesser extent) sauropods probably roamed the Mesozoic Era in crowded herds, and were hunted by sparser packs of large, small and medium-sized theropods.
8. Not all dinosaurs were equally dumb.
It's true, some plant-eating dinosaurs (like Stegosaurus) had brains so tiny compared to the rest of their bodies that they must have been only a little bit smarter than giant ferns. But predatory dinosaurs large and small, ranging from Troodon to T. Rex, had more respectable amounts of grey matter, since they needed better-than-average sight, smell, agility and coordination to hunt down prey. (Let's not get carried away, though--even the smartest dinosaurs were only on an intellectual par with modern ostriches, nature's D students.)
The intelligence of dinosaurs is measured by so-called Encephalization Quotient(EQ). EQ of a crocodile, which is the only close relative of dinosaurs, was set to 1. Dinosaurs with value less than 1 were not very smart, but those with higher values could have been quite intelligent.
9. Dinosaurs lived at the same time as mammals.
Many people mistakenly believe that mammals "succeeded" the dinosaurs 65 million years ago, appearing suddenly on the scene to occupy the ecological niches rendered vacant by the K/T Extinction. The fact is, though, that early mammals lived alongside dinosaurs (usually high up in trees, out of harm's way) for a large part of the Mesozoic Era. Most of these early furballs were about the size of mice, but a few (like the dinosaur-eating Repenomamus) grew to respectable sizes of 50 pounds or so, tiny by T. Rex standards but huge for an ancient mammal.
10. Pterosaurs and aquatic reptiles weren't technically dinosaurs.
It may seem like nitpicking, but the name "dinosaur" applies only to land-dwelling reptiles sporting a specific hip structure (among other anatomical traits). As large and impressive as some genera (such as Quetzalcoatlus and Liopleurodon) were, flying pterosaurs and swimming plesiosaurs, ichthyosaurs and mosasaurs weren't dinosaurs at all--and some of them weren't even all that closely related to dinosaurs, save for the fact that they're also classified as reptiles.