In unicellular organisms functions like digestion, respiration and reproduction are performed by a single cell. Whereas in multicellular animals the same basic functions are carried out by different groups of cells in a well-organized manner.
The body of a simple organism like Hydra is made of different types of cells and the number of cells in each type can be in thousands. The human body is composed of billions of cells to perform various functions. In multicellular animals, a group of similar cells along with intercellular substances perform a specific function. Such an organization is called tissue.
All complex animals consist of only four basic types of tissues. These tissues are organized in specific proportion and pattern to form an organ like stomach, lung, heart, and kidney. When two or more organs perform a common function by their physical or chemical interaction, they together form an organ system, e.g., digestive system, respiratory system, etc. Cells, tissues, organs and organ systems split up the work in a way that exhibits division of labour and contribute to the survival of the body as a whole.