regarding the first one; how are they classified?
Insecta, Pisces, Amphibia, Aves, and Mammalia.
Animal
In 1758, Carl Linnaeus created the first hierarchical classification in his "Systema Naturae". In his original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then the last four have all been subsumed into a single phylum, the Chordata, while his Insecta (which included the crustaceans and arachnids) and Vermes have been renamed or broken up. The process was begun in 1793 by Jean-Baptiste de Lamarck, who called the Vermes "une espèce de chaos" (a chaotic mess) and split the group into three new phyla, worms, echinoderms, and polyps (which contained corals and jellyfish). By 1809, in his "Philosophie Zoologique", Lamarck had created 9 phyla apart from vertebrates (where he still had 4 phyla: mammals, birds, reptiles, and fish) and molluscs, namely cirripedes, annelids, crustaceans, arachnids, insects, worms, radiates, polyps, and infusorians.

what characteristics do all the organisms in the animal kingdom share Animals are eukaryotic, multicellular, heterotrophic, respire aerobically what is meant by the third feature? A heterotroph  is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. can you give example of such species? All animals and fungi, ome bacteria and protists, and many parasitic plants.
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Animal [SEP] History of classification
In the classical era, Aristotle divided animals, based on his own observations, into those with blood (roughly, the vertebrates) and those without. The animals were then arranged on a scale from man (with blood, 2 legs, rational soul) down through the live-bearing tetrapods (with blood, 4 legs, sensitive soul) and other groups such as crustaceans (no blood, many legs, sensitive soul) down to spontaneously-generating creatures like sponges (no blood, no legs, vegetable soul). Aristotle was uncertain whether sponges were animals, which in his system ought to have sensation, appetite, and locomotion, or plants, which did not: he knew that sponges could sense touch, and would contract if about to be pulled off their rocks, but that they were rooted like plants and never moved about.