All organisms, from the bacteria living in an animal's large intestine to the giant sequoia trees in some forests, have a role in an ecosystem, which is defined as a group of interacting organisms and the environment in which they live. Ecosystems have both abiotic and biotic factors. Abiotic factors are the parts of an ecosystem that are not living, like rocks, soil, temperature, light, and water. Biotic factors are the parts of an ecosystem that are alive, such as plants and animals, bacteria, and fungi. Ecosystems can be very large or very small.

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Many ecosystems together fit inside a biome. A biome is a region characterized by things like abiotic factors and plant life. Similar biomes exist in different places all over the planet, as shown on this map. Each color on the map represents the same biome. For example, notice that the northern most areas of North America and Asia have the same two colors. This means that those two biomes exist in both places. Biomes are divided into two large groups. Terrestrial biomes are on land, and aquatic biomes have a large component of water.

The outermost "nesting doll" of ecological terms is called the biosphere. All of Earth's biomes fit into the biosphere, which is defined as all living things on Earth and their interactions with the other three spheres. These are the hydrosphere (water), the lithosphere (Earth's rocky outer layers,) and the atmosphere (layer of gases that surround Earth.)