Dinosaurs are types of reptiles which stopped existing about 65 million years ago. There were many different types of dinosaurs, some of which were extremely large. They lived on the earth 150 million years ago. Some dinosaurs ate lizards, turtles, eggs, or early mammals. Some hunted other dinosaurs or scavenged dead animals. Most, however, ate plants.
We'll never know "exactly" how dinosaurs looked like. But we can make educated guesses based on what we know about them: for example the compsognathus had 60 cm tall, weighted 5,5 kilos and he was a very fast runner; the pteranodon had a very small bodies, he was very light and he had a very long break; the velociraptor was a medium size dinosaur, he weighted 45 kilos and he was strong, fast runner and he had a large claws; the tyrannosaurus was 6 meters tall, he was heavy and he had strong legs and long tail; etc. As we know the dinosaurs disappearing 65 million years ago. The theory A said that a giant meteorite from space hit the earth and huge dust clouds rose up and blocked the sunlight, so all plants died and dinosaurs starved or froze to death. The theory B said that the climate changed a lot because the continents were moving around. Summers got hotter and winters got colder. Dinosaurs had such big problems with these changes that they died. The theory C said that a poisonous plant appeared which killed dinosaur. As a result the meat-eating dinosaurs had no food, so they died.