The Kingdom Animalia (=Metazoa) is one of a handful of lineages rooted far back in the branching tree representing the history of life on Earth. This lineage that is composed of those organisms we know as "animals" represents one of the three major origins of multicellularity (the other two large and diverse groups of multicellular organisms are the fungi and the green plants).
It is difficult to list characteristics that apply to all animals, since various branches of the animal tree have undergone a range of significant modifications. However, most animals obtain energy from other organisms, feeding on them as predators (killing and eating a prey item), parasites (feeding on their "prey" without killing it, at least not immediately), or detritivores (ingesting tiny bits of decomposing organic material such as fallen leaves). In contrast to animals, most plants make their own food, through the extraordinary process of photosynthesis, using energy captured from the sun; most fungi break down decaying organic material (without ingesting it) into its chemical constituents and absorb released nutrients. Animal cells lack a rigid cell wall (some form of which is typical of plants and most fungi) and their cell biology and physiology differ in a variety of other ways from other organisms.
The diversity of animals is impressive. Zhang (2011) recently coordinated an effort to outline a classification scheme for all known animals and to estimate species richness (i.e., number of species) in different parts of the animal tree. A refinement of this work was immediately begun to fill in information gaps, but results from the 2011 publication are enlightening. More than 1.5 million animal species have been described (and many more continue to be discovered and formally described each year). The phylum Arthropoda (insects, spiders, crustaceans, etc.) accounts for around 80% of this total; around 2/3 is accounted for by the insects alone. Well over a third of all known insects (and around a quarter of all known animal species!) are beetles: nearly 400,000 different species of beetles have already been described. More than 40,000 spider species and over 50,000 species of Acari (mites and ticks) have been described. Nearly 70,000 species of Crustacea (crabs, shrimps, barnacles, pillbugs, and many groups unfamiliar to those who don't study them!) are known. The Myriapoda (millipedes, centipedes, and relatives) includes around 12,000 desribed species. The Mollusca (clams, snails, octopuses, and relatives) is among the largest animal phyla, with nearly 120,000 known species. There are over 17,000 known species of Annelida (segmented worms, including earthworms, "polychaete" worms, and their relatives), Nearly 120,000 Hymenoptera Nearly 160,00 Lepidoptera. Even some groups most people have never even heard of are quite diverse. For example, there are over 1000 described Acanthocephala, over 3000 Pseudoscorpiones, and more than 1500 Rotifera species (and rotifer specialists believe this last number may represent just a tenth or less of the true global rotifer species diversity. By comparison with these invertebrate clades, the generally more familiar vertebrate groups are less diverse, but many people may still be surprised to learn, for example, that there are around 32,000 species of described "fishes" and nearly 6,000 desrcibed mammal species. The numbers presented here are just an appetizer. Anyone seriously interested in biodiversity will enjoy studying the original volume by Zhang and colleagues which is a href="freely">http://www.mapress.com/zootaxa/list/2011/3148.html">freely available online.