The Mesozoic Era was a huge geologic era that extends from 251 to 65 million years ago, for a total length of about 186 million years. It is preceded by the Paleozoic and followed by the Cenozoic. Mesozoic means "middle animal life", as it refers to the middle era of animal life. Except for the very beginning of the Mesozoic, the Earth was mostly warm and humid, with high sea levels and continental arrangements substantially different than today's. Forests extended to high latitudes in the north and south, and the polar ice caps of today were mostly absent.
The Mesozoic is appropriately known as the Age of Reptiles or Age of the Dinosaurs, though dinosaurs didn't evolve until about 20 million years into the period. The Mesozoic began after the worst mass extinction in the history of the planet, the Permian-Triassic extinction, which wiped out 98% of marine genera and 70% of terrestrial vertebrate genera. Before the extinction, the ancestors of mammals, called synapsids, were the dominant terrestrial creatures, but just a few million years into the Mesozoic, reptiles -- especially archosaurian reptiles — began to rapidly diversify and take over ecosystems. This event is known as the Triassic takeover.
Mesozoic animals included the early dicynodonts, pig-sized tusked animals that are the distant ancestors of mammals and used to be called mammal-like reptiles even though they are nothing like reptiles. Dicynodonts evolved into the pig to ox-sized kannemeyeriidae, stocky Mesozoic animals that were the dominant herbivores throughout the Triassic period, which is the beginning of the Mesozoic. Other Mesozoic animals from the earlier period included the rhynchosaurs, small and abundant reptilian herbivores with sharp beaks, and the early archosaurians, which started to race ahead of synapsids because of their faster progress towards erect limbs, which conserve energy, and their ability to conserve water better.
Of course, the dominant Mesozoic animals were the dinosaurs and their relatives, like pterosaurs (winged reptiles) and plesiosaurs, ichthyosaurs, and mosasaurs (marine reptiles). Appearing about 230 million years ago as small bipedal predators, dinosaurs had a slow start until the Triassic-Jurassic extinction event, which wiped out competitors such as aetosaurs, ornithosuchids, phytosaurs, and rauisuchians, leaving the world wide open for dinosaur radiation and dominance. Some of the famous dinosaur groups that wandered the world for most of the Mesozoic include the theropods (which includes all dinosaur predators), armored dinosaurs like Ankylosaurus, assorted small herbivorous dinosaurs, and the gigantic sauropods, the largest of which rivaled even the blue whale in size. Many more Mesozoic animals remain to be discovered, even though we currently recognize many tens of thousands of species.