The Kingdom Monera is a catch-all top-level classification for the prokaryotic organisms that was in common usage during the mid to late twentieth century. The Monera consists of those organisms that have their genetic material loose in the cell rather than packed into a cell nucleus, as is seen in the cells of eukaryotic organisms such as plants and animals.
On the more recent classification system by Woese et al (1990), constructed in the light of new genetic data, the Kingdom Monera has been replaced by two Domains called Archeae and Bacteria, and the Domain is considered to be the top-level classification in the system. The Eukaryotes in general form the only other Domain identified on this model, so this is known as the Three Domain System in biological classification.
The Archeae, which were formerly known as the archaebacteria, consist of a group of prokaryotes that are considered to be the very oldest living things on the planet, and which often thrive in the harshest of environments. They differ from those in the Bacteria Domain on the new classification system in terms of their membrane lipid structure as well as in their cell walls and transfer RNAs.
An example of the Archeae is the Halophiles, which thrive in extremely salty conditions. Thermophiles, meanwhile, are microorganisms that grow best above 45 degrees Celsius. Thermoacidophiles thrive in conditions that are not only high in temperature but also highly acidic. In a further example, the methanotrophs are microorganisms that thrive in methane rich environments.
The other Domain that has been posited in the new classification system, and is derived from the Kingdom Monera, is the Bacteria. This type of organisms has a distinct lineage descended from an incredibly ancient common ancestor with the Domain Archeae. Perhaps somewhat surprisingly it has even been found that the Bacteria are further away in genetic terms from both the Eukaryotes and the Archeae than these latter two are from each other.
The key distinction in identifying bacteria is drawn between those bacteria that have no outer membrane, and are known as gram-positive bacteria, and those that are surrounded by an outer membrane, and are known as gram-negative bacteria. The phylum actinobacteria is an example of the gram-positive bacteria and the phylum spirochaetes is an example of the gram-negative bacteria. However, there are many phyla of bacteria that are considered ungrouped along these lines, such as cyanobacteria, for example.