The Monerans are the most numerous and widespread organisms on Earth.
They comprise the only kingdom of prokaryotic organisms, those which lack a nucleus or other membrane-bounded organelles. External to the plasma membrane, most bacteria have a cell wall partially composed of peptidoglycan, a complex structural molecule not found in eukaryotic cells.
Kingdom Monera is a biological classification, and consists entirely of the bacteria - very small one-celled organisms. To get an idea of just how small bacteria are, take a look at the width of a millimeter - the smallest units on the metric side of a ruler. A thousand bacteria can sit side by side in just 1 tiny millimeter! Despite their small size, bacteria are the most abundant of any organism on Earth. And they're everywhere! Bacteria can be found in the air, soil, water, on you, and inside you. In fact, there are more bacterial cells inside your gut and on your skin than there are cells in your entire body - no matter how many times you try to wash them off.
The Columbia Encyclopedia defines Monera as a...
taxonomic kingdom that comprises the prokaryotes (bacteria and cyanobacteria). Prokaryotes are single-celled organisms that lack a membrane-bound nucleus and usually lack membrane-bound organelles (mitochondria, chloroplasts; see cell, in biology). They have a small ring of DNA as their genetic material and reproduce asexually. A recently proposed system classifies these organisms in two major groupings called Eubacteria and Archaebacteria, or archaea. Monera has been contrasted with the kingdoms of eukaryotic organisms (protists, fungi, plants, and animals...).