Organism Lesson for Kids: Definition & Classification

Instructor: Emily Lockhart

Emily has taught science and has a master's degree in education.

In this lesson you will find that you could classify every living thing on the planet. Living things, called organisms, can be organized into categories from very broad to more specific using levels of classification.

Understanding Organisms

Imagine sitting in the jungle and counting every insect, plant and animal around you. What about identifying every organism on the planet? An organism is any living thing, from the smallest bacteria to the humongous blue whale. The same kind of organisms are called a species, and believe it or not, scientists estimate there are about 8.7 million different species on Earth, with about 75% of them on land and 25% in the ocean. Even more amazing is that only about 1.2 million of these species have been identified. Whew... that is a lot of discovery still out there!

Classifying Organisms

You may have realized that animals of different species (polar bears and black bears, for example) may all look very similar. Scientists of the past noted this too and began using these similar characteristics to put living things into categories in a process called classification. Every organism that has ever lived, even the approximately 7.5 million that have yet to be discovered, can be classified. There is a whole field of science called taxonomy that deals with the identifying, classifying and naming of all organisms. The eight levels of classification from the broadest to the most specific can be seen in the image below:

Levels of classification

All organisms can fit into these levels. The broadest of categories would be all life. The unifying property of all life is DNA. DNA is the chemical instructions that tell each cell what its job is in helping our bodies function.


For the broadest classification level, all organisms are separated into three domains: Archaea and Bacteria domains contain organisms whose cells do not have a nucleus, while organisms in the Eukarya domain have cells that do contain a nucleus. This lesson will focus on the Eukarya domain.

Kingdom, Phylum and Class

There are four kingdoms within the Eukarya domain:

  • Animals
  • Plants
  • Fungi (like mushrooms)
  • Protists (like amoebas)

Each kingdom is further divided into phylum. Let's simplify to just the Animal kingdom and look at a few phylum you might be familiar with. Earthworms are in the Annelid phylum, insects are in the Arthropoda phylum, and the Chordata phylum includes all vertebrates, any animal with a backbone - we are in this phylum.

Each phylum is then divided into class. The classes of the Chordata phylum is divided into Fish, Mammals, Amphibians, Reptiles and Birds.

Let's look at the Mammal class, which includes anything that has hair and nurses their young. Some mammals eat meat, while some eat plants.

Kingdom, Phylum and Class examples from the Eukarya domain

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