Domain Bacteria:  Bacteria are microscopic and unicellular, with a relatively simple cell structure lacking a cell nucleus, and organelles such as mitochondria and chloroplasts.  Bacteria are the most abundant of all organisms. They are common in soil, water, and as symbionts of other organisms. Many pathogens are bacteria. Most  only 0.5-5.0 μm in their longest dimension, although giant bacteria may grow past 0.5 mm in size. They generally have cell walls, like plant and fungal cells, but with a very different composition. Many move around using flagella, which are different in structure from the flagella of other groups.

Domain Archaea:  Archaea are single-celled organisms lacking nuclei. They were originally described in extreme environments, but have since been found in all types of habitats.  They have a single cell membrane that lacks a peptidoglycan wall. Further, both bacteria and eukaryotes have membranes composed mainly of glycerol-ester lipids, whereas Archaea have membranes composed of glycerol-ether lipids. These differences may be an adaptation on the part of Archaea to hyperthermophily. Archaeans also have flagella that are notably different in composition and development from the superficially similar flagella of bacteria.

Domain Eukarya:  This is an organism with complex cells, in which the genetic material is organized into membrane-bound nuclei.  Eukaryotic cells are much larger than prokaryotes, typically a thousand times by volume. They have a variety of internal organelles, and a cytoskeleton composed of microtubules and microfilaments, which play an important role in defining the cell's organization. Eukaryotic DNA is divided into several bundles of chromosomes. In addition to asexual cell division, most eukaryotes have some process of sexual reproduction via cell fusion, which is not found among prokaryotes.