1. Bacteria: This domain includes most of the prokaryotes we are familiar with, from those found in soil and water to those living in extreme environments.

2. Archaea: This domain consists of single-celled organisms that were originally classified as bacteria but are now recognized as distinct from them due to their unique biochemistry, genetics, and physiology. Many archaea thrive in extreme environments like hot springs, salt lakes, and hydrothermal vents.