A group of cells that performs a similar function is known as a tissue. Multicellular organisms such as animals all contain differentiated cells that have adapted to perform specific functions. These differentiated cells group together to form tissues. A group of tissues that perform similar functions are known as organs.
As multicellular organisms develop, their cells differentiate to carry out different functions. As they differentiate into different groups, they form into tissues. In total there are four basic tissue types: connective tissue, muscle tissue, nervous tissue and epithelial tissue. All these tissue types group together to form organs and organ systems. Connective tissues support, protect and strengthen the organism. Muscle tissues are broken into three different types of muscle known as skeletal muscle, smooth muscle and cardiac muscle. All types of muscle tissue contract and relax to help organisms move and function. Nervous tissue transmits electrical and chemical signals throughout the organism. These signals allow cells to communicate with each other. Epithelial tissues form the surface of the skin of many organisms and line the cavities of organs and organ systems. Each tissue type performs specific functions in specific organisms. Their functions depend on the cell type that the tissues are comprised of and the amount of cells present.