* Bacteria: These are single-celled prokaryotes (lacking a nucleus) that are incredibly diverse and found in almost every environment on Earth.

* Archaea: Also single-celled prokaryotes, archaea were initially thought to be bacteria. They are now recognized as distinct due to their unique biochemistry and ability to thrive in extreme environments (like hot springs or highly saline areas).

* Eukarya: This domain includes all organisms with eukaryotic cells, which have a nucleus and other membrane-bound organelles. Eukarya encompasses a wide range of life forms, including plants, animals, fungi, and protists.