Animals are classified in a variety of ways. This helps scientists to study the relationships in animal groups and to see the whole animal family tree as it has developed through time. The study of animal classification is called taxonomy.

The basic unit of an animal is the cell. A cell is the smallest unit of any animal or plant. Some animals are one celled, some consist of millions of cells. Each cell is filled with a living matter called protoplasm. It also has a nucleus that is the center of the cell and directs its activities. The cytoplasm is the area outside of the nucleus. Each cell is held together by a cell membrane which is like a very thin skin for the cell. Many cells have different jobs to do in an animal, whether they be bone, blood, skin cells.

A group of the same kinds of cells are called tissues. A group of tissues that work together to do a job in the animal's body is an organ. The stomach, heart, kidneys, lungs are examples of organs. A group of organs that do a number of jobs of the same kind are systems.