Dinosaurs lived on Earth during a period called the Mesozoic Era which included three periods: Triassic, Jurassic, and Cretaceous.  The beginning of this history span can be dated back to roughly 230 million years ago with dinosaurs walking the Earth for over 165 million years.  Dinosaurs are classified as reptiles.  Similarly, the Mesozoic Era is known as the Age of Reptiles.  There were two types of dinosaurs, meat-eaters and plant-eaters.  This is important as it has a direct relationship to the physical characteristics of the dinosaurs.  Meat-eaters had hooked claws on their toes and plant-eaters with hooves.   There is no definitive number regarding the different species and many dinosaur fossils are still being uncovered by paleontologist, a person who studies dinosaurs.  Some of the most well-known dinosaurs include: Tyrannosaurs, Velociraptors, Triceratops, Stegosaurus, Brachiosaurus, Dilophosaurus, and many more.