Bacteria are prokaryotic cells which lack membrane bound organelles. Eukaryotic cells, which comprise the other Kingdoms: Animalia, Plantae, Fungi and Protista, on the other hand, contain specialized organelles like a nucleus, mitochondria, ribosomes, etc. which are surrounded by membranes. Bacterial cells are about 1/10 the size of eukaryotic cells. Archaea inhabit many harsh environments on Earth, like in hot springs, or deep below the surface of the ocean on the sea floor. Eubacteria can be found on or in living things and all over the biosphere. It is thought that most bacteria haven't yet been identified. There are more bacterial cells in a human body, then human cells. They are very successful life forms that date back to the beginning of life on Earth.