Dinosaurs were a highly diverse reptile group which existed between about 245 and 66 million years ago and their fossils were reported from all continents. They were the most dominant and characteristic terrestrial vartebrates during the Mesozoic. We know more than 1000 dinosaur species from fossils, but probably there were much more species which did not fossilized or just we did not find their fossils yet.
Dinosaurs taxonomically belong to archosaurs, which is a clade of Diapsida, and also includes birds and modern crocodilians. Dinosaurs have two basic divisions: Ornithiscia ("bird-hipped" dinosaurs) and Saurischia ("lizard-hipped" dinosaurs). Saurischia includes the Herrerasauria (bipedal carnivores, one of the earliest dinosaur groups), Theropoda (mostly bipedal carnivoeous dinosaurs) and Sauropodomorpha (long-necked herbivorous dinosaurs) clades. Ornithischia consists of three clades; Heterodontosauridae (early ornithopods with canine-like teeth), Thyreophora (armoured dinosaurs: Stegosauria and Ankylosauria), and Neornithischia (Ornithopoda and Marginocephalia).
Dinosaurs of the Wessex formation. Source: By ABelov2014 https://abelov2014.deviantart.com/ - https://abelov2014.deviantart.com/art/Barremian-fauna-660152146, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=64876262
Those further ancient reptiles, like mosasaurs, pterosaurs, ichthyosaurs, and plesiosaurs, which are often popularly thought to be dinosaurs, too, does not belong taxonomically to the dinosaurs and consequently they are members of totally different reptile groups.
Dinosaur fossils were recognized for thousends of years, although people did not realized what they really were. For example, in ancient China they were thought to be dragon bones, and in Europe they were considered to be remnants of giants. The first scientific descriptions about these fossils, not knowing their true nature yet, were presented during the late 17th century, in England. The first two dinosaurs, Megalosaurus and Iguanodon, were scientifically described in the early 19th century by William Buckland and Mary Ann Mantell. Soon, these creatures become of great interest to the scientific society of the world, and the name „dinosaur” was created for them by Sir Richard Owen. Until William Parker Foulke discovered the first nearly complete dinosaur skeleton in North America, which belonged to a Hadrosaurus, most scientist believed that dinosurs used all their four feet for walking, but this new finding was clearly a bipedal animal. In 1969 John Ostrom scientifically described Deinonychus, which was supposed to have active hunting life-style, and made scientists consider the warm-blooded nature of dinosaurs.
Dinosaur fossils were reported from all continents. During the Mesozoic, they conquered and succesfully adapted to all terrestrial habitats. They showed a great variability in their body sizes – the smallest species, found trapped in a 99 million years old amber piece, had the same size like a hummingbird, while the largest ones, like Argentinosaurus, may have weighed 100 000 kilograms and its length reached 30 to 40 meters.
Size and weight comparison of different dinosurs. Source: By MathKnight and Dr. Zachi Evenor - Own work, based on File:Biggest-Dinosaurs-ver07-he.svg, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=71674409 - Modified by editor
They were a very diverse group from morphological and ecological standpoints, too.
Some dinosaurs were carnivors, others herbivors, and there were also omnivores, fish-eaters, insectivores and seed-eaters amongst them. According to our present knowledge, all dinosaur species layed eggs, and many of them build nests as well. Some dinosaur groups showed well developed social behavioural patterns, like herding or flocking, hunting in packs, territorial behaviour, parental care, and using visual signs for communication.
Before the second half of the 20th century, most scientists believed that dinosaurs were cold-blooded animals, just like all the today living reptiles. However, most of the new scientific results since the 1970s suggest that they were warm-blooded with active life-style, elevated metabolism and were able to maintain their body temperature higher than their environment.
Dinosaurs, alongside with many other animal groups like ammonites, pterosaurs and ichthyosaurs, suddenly become extinct approximately 65 million years ago, during the Cretaceous-Paleogene extinction event. Most scientists agree that this extinction event was triggered by a meteorite impact, but other factors like massive volcanic activity also may have contributed to it. However, some fossils suggest that a handful of dinosaurs populations survuvived the extinction event and existed for about further half a million years in early Paleocene.
Tyrannosaurus rex skeleton. Source: pixnio.com. Author: pics_pd. https://pixnio.com/architecture/museums/dinosaur-skeleton-bone-fossil-museum-dinosaur-triceratops. Licence: CC0 - https://creativecommons.org/licenses/publicdomain/