A prokaryote is an organism consisting of one or more prokaryotic cells. Bacteria are the best known and most studied form of prokaryotic organisms, although the discovery of a second group of prokaryotes, called archaea, has provided evidence of a third cellular domain of life and new insights into the origin of life itself.
Prokaryotes are unicellular organisms that don't develop or differentiate into multicellular forms. Some bacteria grow in filaments, or masses of cells, but each cell in the colony is identical and capable of independent existence. The cells may be adjacent to one another because they did not separate after cell division or because they remained enclosed in a common sheath or slime secreted by the cells. Typically though, there is no continuity or communication between the cells.
Prokaryotes are capable of inhabiting almost every place on the Earth, from the deep ocean, to the edges of hot springs, to just about every surface of our bodies. Compare with eukaryotes.