The best way to tell apart the two main kinds of dinosaurs is hip structure. One major group of dinosaurs (ornithischians) is characterized by a bird-like hip (ornitho = bird, ischium is a hip bone), and the other major group of dinosaurs (saurischians) has a lizard-like hip (saur = lizard).

Somewhat confusingly, one group of saurischian dinosaurs developed a bird-like hip independently of the bird-like hip in ornithischians. This unusual group of saurischians with bird-like hips includes, perhaps no surprisingly, birds, and their closest relatives, including small-bodied, meat-eating dinosaurs such as Velociraptor. This is one of the major reasons scientists think that birds ARE a kind of dinosaur.

Dinosaurs are generally classified based on their bone structure. In particular, the hip structure is a distinguishing feature of dinosaurs that allows them to quickly be sorted into two groups, each of which contain many sub-groups.

There are many different kinds of dinosaurs, and they are different from each-other in different ways.

Since the only living dinosaurs are birds, it is easiest to imagine the differences that distinguish different kinds of birds. Extinct species of dinosaur probably were different in similar ways.