The great majority of dinosaurs have been grouped into two main orders, the Ornithischians and the Saurischians. They have been grouped based on key features shared by the dinosaurs. The orders have then been subdivided into smaller groups known as suborders and infraorders.

The Ornithischians share the common feature of being plant eaters and having a backward-pointing pubis like a bird. They share other features as well, including a horned beaks at the front of the mouth for snipping plants, and leaf-shaped cheek teeth. The adaptations occurring in Ornithischians was wide, producing such dinosaurs as the triceratops which had three large horns on the head, and the ankylosaurus whose upper body was totally covered in armour. The Ornithischian order is subdivided into several suborders listed below.

The Saurischians share the common feature of a forward-pointing pubis (hip bone).