A tissue in biological systems is a group of interconnected cells that perform a specific function. Tissues are classified into four primary types: epithelial, connective, muscle, and nervous tissues. Each type has distinct structures and roles. For example, epithelial tissues cover surfaces and protect organs, connective tissues provide support and structure, muscle tissues enable movement, and nervous tissues transmit signals. Understanding tissues is vital in fields like medicine and biology, as they are fundamental to the organization of living organisms and the understanding of various health conditions.