One of the problems with explaining the scientific definition of the word "dinosaur" is that biologists and paleontologists tend to use much drier, more precise language than your average dinosaur enthusiast on the street. So while most people intuitively describe dinosaurs as "large, scaly, dangerous lizards that went extinct millions of years ago," experts take a much narrower view.
In evolutionary terms, dinosaurs were the land-dwelling descendants of ancient archosaurs, egg-laying reptiles that survived the Permian/Triassic extinction event 250 million years ago.
Technically, dinosaurs can be distinguished from the other creatures descended from archosaurs (pterosaurs, the flying reptiles, and prehistoric crocodiles) by a handful on anatomical quirks. Chief among these was posture: dinosaurs had either an upright, bipedal gait (like modern birds), or, if they were quadrupeds, a stiff, straight-legged style of walking (unlike modern lizards, whose limbs splay beneath them). Beyond that, the distinguishing anatomical features become rather arcane; try an "elongate deltopectoral crest on the humerus" on for size.
Saurischian and Ornithischian Dinosaurs
For the sake of convenience, the dinosaur family is divided into two main groups. To vastly simplify the story, starting about 250 million years ago a subgroup of archosaurs split off into two types of dinosaurs, distinguished by the structure of their hip bones. Saurischian ("lizard-hipped") dinosaurs went on to include predators like Tyrannosaurus Rex and huge sauropods like Apatosaurus, while ornithischian ("bird-hipped") dinosaurs consisted of a diverse assortment of other herbivores, such as hadrosaurs, ornithopods and stegosaurs.
(Confusingly, we now know that birds descended from saurischian, rather than ornithischian, dinosaurs!)
You may have noticed that the above definition of dinosaurs refers to "land-dwelling" reptiles, which technically excludes marine reptiles like Kronosaurus, and flying reptiles like Pterodactylus from the dinosaur umbrella. Also occasionally mistaken for true dinosaurs are the large therapsids and pelycosaurs of the Permian period, like Dimetrodon and Moschops. While some of these ancient reptiles could have given the average Deinonychus a run for its money, rest assured that they weren't allowed to wear "Dinosaur" name tags at the keg parties of the Jurassic period!
