Here's a breakdown:

* Cells: The basic unit of life.

* Tissues: Groups of similar cells that work together to perform a specific function.

* Organs: Structures made up of different tissues that work together to perform a complex function.

* Organ Systems: Groups of organs that work together to carry out major bodily functions.

For example, the digestive system is made up of organs like the stomach, intestines, and liver, all working together to break down food and absorb nutrients.