Dinosaurs were no monsters.They were quiet normal animals, which fit into the spectrum of living beings. Today there live a lot of different kinds of animals in the world. Biologists catogorized them according to the similarities in physiology and physiognomy into families of related species.
This is a rough picture of what we see today. Since Charles Darwin we know, that all forms of life developed from a common origin. The classification above therefore is a snapshot of a continuing evolution process. In the future new forms of life will arrive, other will vanish. This is what happened to dinosaurs. At some time in history they vanished. To understand what dinosaurs are and why they vanished, we look at the tree of life, which shows the development of animals.We concentrate on a subtree, which leads to birds, lizards and crocodiles.
The x-axis of the diagram displays the relation between different species. So why can see, that some dinosaurs were very close to lizards, others to birds or crocodiles.They all developed from one lizard like dinosaur, whose ancestors were small amphibic animals.
The y-axis displays the time,from present (on top) to the beginning of life (on bottom). It is astonishing to see, that nearly all dinosaurs vanish from earth at the same time, the boundary between of cretaceous and tertiary (C-T). Such an exceptional mass extinction is only explainable by a large catastrophic event.