Bacteria are found nearly everywhere on Earth and have evolved into hundreds of thousands of species over billions of years. They are classified into groups based on their structure, physiology, composition, and staining properties. The two main bacterial kingdoms are Archaebacteria and Eubacteria, which include extremophiles that thrive in extreme environments like hot springs, salt lakes, and acidic ocean vents. Common pathogenic bacteria include Staphylococcus and Streptococcus, which can cause infections like acne, strep throat, and food poisoning.