1. Bacteria: These are single-celled prokaryotes, meaning they lack a nucleus and other membrane-bound organelles. They are incredibly diverse and found in almost every environment on Earth.

2. Archaea: Also single-celled prokaryotes, archaea are similar to bacteria in appearance, but they have distinct biochemical and genetic differences. They often live in extreme environments like hot springs, salt lakes, and deep-sea vents.

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