Non-avian dinosaurs, typically referred to as just dinosaurs, are an extinct group of reptiles from the clade Dinosauria. Dinosaurs first appeared between about 243-233 million years ago, and became the most common vertebrates on earth around 201.3 million years ago after the mass-extinction event marking the boundary between the Triassic period and the Jurassic period. Birds, or avian dinosaurs, are the only living descendants of the dinosaurs, which majorly went extinct about 66 million years ago during another extinction event that happened between the Cretaceous period and the Paleogene period. In order to describe and classify them better, dinosaurs are usually split into two groups: the Saurischia, or lizard-hipped dinosaurs, and the Ornithischia, or bird-hipped dinosaurs.
The saurichian (from the Greek for "lizard hip joint") dinosaurs all had hipbones that separated frontwards and backwards at the bottom, the same as those of today's modern lizards and most other reptiles. This pelvic structure, with pubic bone facing down and to the front, was what gave many dinosaurs the ability to walk on two legs, also called bipedalism. The saurichia include all the carnivorous dinosaurs, one of the two primary groups of herbivorous dinosaurs, and, strangely enough, all of the birds.
The theropoda, or theropods (from the Greek for "wild beast foot"), are a group of saurischian dinosaurs characterized by having hollow bones and three-toed arms and legs. Theropods first appeared about 231.4 million years ago, and while they largely went extinct about 66 million years ago, they are the direct ancestors of today's modern birds. Like birds, theropods walked on two legs and had relatively small arms. A few theropods you might recognize are: Tyrannosaurus rex, Velociraptor, Giganotosaurus, Megalosaurus, and Spinosaurus.
The sauropoda, or sauropods (from the Greek for "lizard foot"), are the other main group of saurischian dinosaurs. Sauropods are characterized by their long necks and tails; small heads; and four thick legs. Sauropods were herbivorous and some species grew to enormous sizes. A few well-known sauropods are: Argentinosaurus (the largest of all dinosaurs, measuring up to 40 m in length!), Brachiosaurus, Diplodocus, and Brontosaurus.
The ornithischian (from the Greek for bird hip joint) dinosaurs had hipbones that pointed backwards, much like those of today's birds. The ornithischia include mostly herbivorous dinosaurs, with the addition of a few species that were possibly omnivorous. Paleontologists have found evidence that ornithischian dinosaurs likely lived in groups or herds, and some even had pelts of something that resembled modern day bird feathers.
The thyreophora (from the Greek for "shield bearer"), often called the armored dinosaurs, are a group of ornithischian dinosaurs that had thick plates of body armor. Thyreophora were mostly herbivorous and had small brains relative to their overall size. Stegosaurus and Ankylosaurus are both thyreophora.
The cerapoda, or cerapods (from the Greek for "horned feet), are the other main group of ornithischian dinosaurs. Some well-known cerapods are Triceratops and Iguanodon.