* Bacteria: These are single-celled prokaryotes (organisms without a nucleus). They are incredibly diverse and can be found in almost every environment on Earth.

* Archaea: These are also single-celled prokaryotes, but they are more closely related to eukaryotes (organisms with a nucleus) than bacteria. They often live in extreme environments, like hot springs or salty lakes.

* Eukarya: This domain includes all organisms with a nucleus, including protists, fungi, plants, and animals. This is the domain that humans belong to.

These three domains represent the highest level of classification in the current biological system.