Chelonians are a moderate-sized group of animals belonging to the reptile class (Class Reptilia) and forming their own order, the Order Chelonia. This in turn is related to the other reptilian groups, living and extinct: the crocodiles, lizards and snakes, and tuatara (an endangered lizard-like creature from New Zealand) of our present age, plus the dinosaurs and other prehistoric reptiles. Despite fleeting similarities, chelonians only very distantly related to such dinosaurs as the heavily armoured Ankylosaurus.
Chelonians can normally be characterised by a dry, watertight and scaly skin, and primarily of course by their shell, the bony structure that encases their trunk and from which protrudes their limbs, tail and head. Contrary to popular belief, most chelonians are carnivorous: most tortoises are herbivores, but terrapins and turtles are normally carnivorous, and sometimes extremely predatory. There is diversity amongst chelonians in size, colouring and habitat, in some ways even more so than among lizards: the leatherback turtle, for example, lives in the ocean and makes a 1,400 mile journey between the eastern seaboard of the USA and Ascension Island, while several species live in desert and some in jungle. None are found at high altitude, however.
There are two further points to be noted about chelonians. Firstly, they lead long lives. This is seen at its extreme in the tortoises, which can easily reach 80-100 years of age when properly cared for. The oldest living tortoise in England was, at the time of writing, believed to be 146, while a Galapagos Giant Tortoise that died in the 1990s was believed to have been alive at the time Darwin landed on its home island. Secondly (and obviously relevant to the above), chelonians are not the easiest reptile pet to look after. If you want an easier reptile, choose a lizard or a snake.