Final answer:
Prokaryotes are predominantly unicellular organisms that lack a nucleus and are classified within the domains Bacteria and Archaea.
Step-by-step explanation:
Most prokaryotes are unicellular. Prokaryotic cells, which include bacteria and archaea, are simple, single-celled organisms that lack a nucleus and other membrane-bound organelles. They were the earliest to evolve and are still prevalent today, being found everywhere with sufficient moisture and living on and inside other living things. These organisms are responsible for nutrient recycling on Earth and have been here long before the appearance of multicellular life.