From Citizendium, the Citizens' Compendium
Prokaryotes are primitive single celled organisms whose name originates from the Greek pro, before, and karyon, meaning nut or kernel. Prokaryotic cells are distinctively simpler than eukaryotic cells, lacking a distinct membrane bound nucleus and membrane bound organelles. However, they are also vastly successful. The weight of all prokaryotes is about ten times that of the weight of all eukaryotes, despite their small size. Prokaryotes can live in environments that eukaryotes would never be able to, due to extreme temperatures or ion contents. Most prokaryotes are bacteria; however, another type known as archea inhabit extreme environments. Historically, these made up the kingdom Monera. However, modern study of RNA indicates that bacteria and archea should considered to be two domains, where eukaryotes make up the third domain.
Nutrition and Metabolism
Cambell, NA and JB and Reese. Biology. 6th ed. New York: Benjamen Cummings, 2002. ISBN 0-8053-6624-5.