A tissue is defined in biology as a group of specialized cells that work together to perform a specific function within a multicellular organism. Tissues provide the basic structural level of organization that forms organs and systems. Different body systems rely on various tissue types, each structured for distinct biological roles. Major classifications include epithelial tissue, which covers body surfaces and lines internal cavities; connective tissue, which provides structural support and connects various parts of the body; muscle tissue, which is responsible for movement; and nervous tissue, which facilitates rapid communication and signal transmission throughout the organism. The differentiation and arrangement of these tissues are fundamental to the development and maintenance of life.