Bacteria are tiny, single-celled organisms that can be found almost everywhere on Earth, including soil, water, and even inside the human body. They are prokaryotes, meaning they lack a nucleus and other membrane-bound organelles. Bacteria play essential roles in various processes, such as decomposing organic matter and aiding in digestion. Some bacteria are beneficial, like those in the human gut that help break down food, while others can cause diseases. They reproduce quickly through a process called binary fission, where one cell divides into two. Understanding bacteria is crucial for fields like medicine and environmental science.