Kingdom Monera
Of the five kingdoms (Monera, Protista, Plantae, Fungi and Animalia) Monera is the only "kingdom" containing prokaryotic organisms. Remember that the classification we are using is outdated and that "Kingdom Monera" is now 2 domains, the Archaebacteria and the Eubacteria.
Prokaryotic cells are characterized by having no membrane bound organelles. This means they have no nucleus or any other specialized sections of the cell. Due to this lack, these cells are limited by the rate of diffusion across the cell to obtain nutrients and other necessities. As a result, these organisms are single cells and very small.
These cells can be compared to more advanced cells, the eukaryotic cells, which have membrane bound organelles. These organelles are specialized areas within the cell that are partitioned via sections of cell membrane. The specialization allows for movement other than simple diffusion across the cell, further allowing for an increase in cell size, and ultimately to the specialization of groups of cells (tissues), giving rise to multicellular creatures.