Here's the breakdown:

* Cells: The basic building blocks of life.

* Tissues: Groups of similar cells that work together to perform a specific function. For example, muscle tissue contracts, nervous tissue transmits signals, and epithelial tissue forms coverings.

* Organs: Structures made up of different tissues that work together to perform a more complex function. For example, the stomach is an organ made of muscle tissue, epithelial tissue, and connective tissue to digest food.

* Organ Systems: Groups of organs that work together to perform a major bodily function. For example, the digestive system includes the mouth, esophagus, stomach, intestines, and other organs to break down food.

So, the hierarchy goes: cells -> tissues -> organs -> organ systems.