Bacteria are single-celled organisms with a simple structure. They have a protective outer layer called the cell wall, which helps maintain their shape and provides protection. Inside, bacteria contain cytoplasm, where essential processes occur, and ribosomes for protein synthesis. Some bacteria also have a gel-like substance called capsule that helps them evade the immune system. Bacterial cells typically have a single circular strand of DNA located in a region called the nucleoid. Many bacteria possess flagella, which are tail-like structures that aid in movement. Additionally, some bacteria have pili, which are hair-like appendages that help them attach to surfaces and exchange genetic material.