Introduction to Zoology
Zoology is the branch of biology that includes the study of animals and animal life.
Animals are heterotrophic. That means they are organisms that must rely on other organisms (living or dead plants or other animals) to provide the energy for life. Plants, by contrast, are autotrophic, they are organisms that produce their own food from sunlight and other inorganic sources.
Animals further have sense organs or tissues, which allow them to interact with their environment. This distinguishes animals from fungi (mushrooms and relatives), which are also heterotrophs.
By the requirement that animals have sense organs or tissues, this means they must also be multicellular (having more than one cell in their body). This separates the animals from the Protista, the single celled heterotrophs.
Zoology as a discipline includes, but is not limited to, the study of the behavior, ecology, anatomy, physiology, evolution, and classification of animals. Any study that involves animals as it's primary focus can be considered a part of zoology.
In addition to being divided along the lines mentioned above, zoology is often divided by the organisms being studied. The broadest division by organism is the splitting into categories by whether or not the animals studied have a backbone or spinal column, which may be made of cartilage or bone. Each of those divisions would naturally include their behavior, ecology, anatomy, etc.
Using that division, zoology can be subdivided into two large categories:
Briefly, an invertebrate lacks a spinal column (backbone). Animals in this category include species of the following: protozoans, annelids, cnidarians, echinoderms, flatworms, nematodes, mollusks, and arthropods. Invertebrates make up 90% of the Earth's animals. They range in size from minute to extremely large. Some, like insects, have an exoskeleton, which is a support structure like our own skeleton, but is on the outside of the animal. Others have hard internal support structures, some have no hard parts at all.
A vertebrate has a spinal column or backbone. Animals in this category includes the animals in the subphylum Vertebrata: fish, amphibians, reptiles, birds, and mammals. These are all characterized by a segmented spinal column and a distinct differentiated head. These animals possess internal skeletons which include the vertebral column which at least partially encloses the spinal cord and a skull which houses the brain. When people think of animals, this is often the only kind they think of. Largely this is probably due to the fact that the category includes humans.