What is a group of cells that carry out a certain job called?

A group of cells that carry out a certain job is called a tissue. Several tissues working together form an organ, and several organs together make up an organ system.

Four types of tissues exist in the human body. Epithelial tissue composes the lining of different parts of the body, protects other tissues and secretes substances like mucus. Connective tissue, which includes rigid bone and blood vessels, supports, protects and binds. Muscle tissue is responsible for the movement of body parts. Skeletal, cardiac and smooth muscle are the three types of muscle fiber. Nervous tissue functions as the communication system and consists of the neurons and glial cells.