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

* Archaea: Like bacteria, archaea are single-celled prokaryotes. They were originally classified as bacteria but were later recognized as a distinct domain due to significant differences in their biochemistry, genetics, and cell structure. They are often found in extreme environments like hot springs, salt lakes, and deep sea vents.

* Eukarya: This domain includes all organisms with eukaryotic cells, which have a nucleus and other membrane-bound organelles. While many eukaryotes are multicellular (plants, animals, fungi), there are also many single-celled microorganisms, like protozoa, yeasts, and algae.

So, while bacteria and archaea are exclusively microorganisms, eukaryotes also include a vast array of microscopic life.