An animal is any creature which is a member of the Animalia kingdom. For most modern biologists, this includes human beings, although there are several qualitative differences which are of primary importance (see anthropology, psychology, and so on).
Animals must eat plants or other organisms to survive. The plants are called "producers" and the animals are "consumers". Animals have complex eukaryotic cells, unlike bacteria which have much simpler prokaryotic cells. Animal cells do not have cell walls, only structural proteins in the cell membrane; this allows the animal cell a degree of flexibility.
The simplest of all animals are sponges, which do not move. Sponges have different cells for different tasks, but these cells are not organized further into tissues or organs. Sponges also do not exhibit symmetry, unlike all other animals.
The next simplest animals comprise a single species, which fall somewhere between sponges and all other animals. This species, Trichoplax adhaerans, is very small, yet has cells organized into different tissues, though it does not have organs.
True Animals are grouped into a sub-kingdom called Eumetazoa. All True Animals have tissues and organs, except for one type called Myxozoa. These have regressed to a single-celled form.
The simplest eumetazons are the Radiata, a loose group that includes jellyfish, corals, and comb jellies. Most of them belong in the phylum Cnidaria, except for the comb jellies, which comprise the phylum Ctenophora. Radiatans have what is called radial symmetry, where they can be divided into two even halves in many planes, like a wheel or a circle. All cnidarians have special cells called cnidocytes, which produce the sting. Comb jellies have no such cells and are harmless to humans.
Animals and people
As God created man in His image, it is of primary importance to distinguish the characteristics of people from animals, such as the eternal spirit, the right to "take dominion" over the natural world of animals and plants, and the prospect of enjoying eternal life. Liberals say that these things are of secondary importance and insist that humans are merely apes.
For biologists who regard their field of study as belonging to physical science, issues of psychology and spirituality take a back seat. They emphasize the physical similarities between the human body and the bodies of other creatures, classifying humans as mammals, specifically, primates.
Perhaps it is their methodological naturalism that leads them to ignore major qualitative differences and to speculate that humans are merely animals with somewhat greater intellectual ability. This vision may be more a predisposition than a scientific conclusion.
Biblical origin of animals
The Book of Genesis specifies both when and why God created animals. The "when" is found in the following passage from Genesis ch. 1, which states that God created aquatic creatures and birds on the fifth day and that on the sixth day He created land animals and then man:
And God said, Let the waters bring forth abundantly the moving creature that hath life, and fowl [that] may fly above the earth in the open firmament of heaven. And God created great whales, and every living creature that moveth, which the waters brought forth abundantly, after their kind, and every winged fowl after his kind: and God saw that [it was] good. And God blessed them, saying, Be fruitful, and multiply, and fill the waters in the seas, and let fowl multiply in the earth. And the evening and the morning were the fifth day. And God said, Let the earth bring forth the living creature after his kind, cattle, and creeping thing, and beast of the earth after his kind: and it was so. And God made the beast of the earth after his kind, and cattle after their kind, and every thing that creepeth upon the earth after his kind: and God saw that [it was] good. And God said, Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth. So God created man in his [own] image, in the image of God created he him; male and female created he them. And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moveth upon the earth. Genesis 1:20-28 (KJV)
The "why" is found in the following passage from Genesis ch. 2, which states that after God had created man, He created land animals and birds in an attempt (ultimately unsuccessful) to provide one of the animals as a help mate for man. In the second chapter of Genesis the alternate creation myth offered then has God creating Woman only after He did not find a helpful mate for man in all the animals. This is different from the first chapter in which the animals were created first and then Man and Woman were both created on the same day:
And the LORD God said, [It is] not good that the man should be alone; I will make him an help meet for him. And out of the ground the LORD God formed every beast of the field, and every fowl of the air; and brought [them] unto Adam to see what he would call them: and whatsoever Adam called every living creature, that [was] the name thereof. And Adam gave names to all cattle, and to the fowl of the air, and to every beast of the field; but for Adam there was not found an help meet for him. Genesis 2:18-20 (KJV)