Bacteria are single-celled microorganisms that can be found in various environments, including soil, water, and even inside animals. They have a wide range of shapes and sizes, but most bacteria fall into two main groups - those with a rigid cell wall made of peptidoglycan (gram positive) or without one (gram negative). Bacteria can be beneficial, neutral, or harmful to humans, depending on the type. They play important roles in digestion and maintaining healthy gut flora, but some types can cause diseases such as pneumonia, strep throat, and tuberculosis.