Bacteria (singular: bacterium) are single celled microorganisms that belong to the prokaryotes. They do not have a nucleus and reproduce asexually through cell division. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods to spirals. Bacteria are ubiquitous in every habitat on Earth, growing in soil, acidic hot springs, radioactive waste, seawater, and deep in the Earth's crust. There are typically 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water. In all, there are approximately five nonillion (5×10^30) bacteria on Earth, forming much of the world's biomass.
Bacteria are vital in recycling nutrients, and many important steps in nutrient cycles depend on bacteria, such as the fixation of nitrogen from the atmosphere. However, most of these bacteria have not been characterized, and only about half of the phyla of bacteria have species that can be cultured in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology. - wikipedia
The Bacteria are a group of Prokaryotic and single-cell microorganisms. In this they are similar to archaea, but these two groups evolved differently, and are classified as different domains in the three-domain system: Bacteria, Archaea, Eucaryota