From simple to complex order, the four main terms referring to the body's organization are cells, tissues, organs and organ systems. Cells is the simplest component in the body of an organism, it is composed of different organelles but a cell functions as a single unit. Tissues are group of similar cells working together to perform a certain function. An organ is a distinct structure composed of two or more tissues. While an organ system are group of organs working together to meet the needs of the body. The different organs systems in the body make up the whole organism or individual.