Here's a breakdown:

* Cells: The basic building blocks of life.

* Tissues: Groups of similar cells working together to perform a specific function (e.g., muscle tissue, nervous tissue).

* Organs: Structures made up of different tissues that work together to perform a specific function (e.g., heart, lungs, stomach).

* Organ Systems: Groups of organs that work together to carry out major bodily functions (e.g., the digestive system, the circulatory system).

For example, the digestive system includes the mouth, esophagus, stomach, small intestine, large intestine, liver, pancreas, and gallbladder, all working together to break down food and absorb nutrients.