Dinosaurs dominated the terrestrial (land) environments for approximately 160 million years. Other orders of reptiles dominated the skies and the seas and are often mistakenly thought of as dinosaurs. But the taxonomic superorder of Dinosauria only contained two specific orders: Saurischia and Ornithischia.
These two taxonomic orders of reptile are defined based on the pelvic bone structure of the multitude of species within them. Saurischian species are described as being "lizard-hipped" while Ornithischian species are called "bird-hipped". While having a similar pelvic structure to the birds of today, the Ornithischia are not the ancestors of our avian companions.
For those people unfamiliar with the taxonomic classification system used by biologists and paleontologists, this may seem a small difference to focus upon. However, it is these small variations and more particularly, how far back they extend, that defines the evolutionary development of all species. Prior to our development of genetics, the classification of species was solely dependent on such small morphological differences. Difficulties could, did and still arise. Environmental effects through natural selection can induce evolutionary changes in species that are genetically disparate, resulting in morphological similarities. Many species that were previously considered to be closely related have been found to be far more distantly related when genetically assessed, while others thought distant have proven closer than suspected. However, at the taxonomic order level, such determinations have generally proven correct and substantive.
All species grouped under the taxonomic order Ornithischia have a pubis bone that points downward and towards the back or tail of the animal. In comparison, those in the order Saurischia have a pubis bone that points down and forward. This defines Ornithischian dinosaurs as having a common ancestor that originated this skeletal formation, with subsequent variations dividing them into families and species over millions of years following that development. Effectively, the taxonomic classification system is a hierarchy of evolutionary development; the higher up the classification scale you go, the further back in time you are traveling. A mutation occurs, if it is successful it is propagated, if it continues to be successful, the animals or plants, etc that have that genetic mutation undergo speciation, resulting in numerous living beings that are related but only able to procreate with those most similar to themselves.
Because this defining characteristic is so far back in the evolutionary history of the species classified under the order Ornithischia, differences between such species can be considerable. But one thing that all species under the umbrella of the taxonomic order of Ornithischia have in common, is that they were all herbivores (plant eaters). In addition, their skulls had small openings between the orbits (eye sockets) and nares (nostrils) called antorbital fenestrae, that their dinosaur cousins of the order Saurischia did not.
Beside these similarities, the vast array of Ornithischian dinosaurs had a multitude of variations, including both two- and four-legged species. They could be small or large, fast or slow and heavily shielded/armored or not. The duck-billed dinosaurs, the hadrosaurs and lambeosaurs, were Ornithischian, as were the plated and armored stegosaurs. The herbivorous Ornithischians outnumbered all of the herbivorous, omnivorous and carnivorous Saurischian dinosaurs by a considerable amount, possibly by as much as ten to one on a species basis alone.