Here's a breakdown:

* Cells are the basic building blocks of life.

* Tissues are groups of similar cells that work together to perform a specific function.

* Organs are made up of different types of tissues that work together to perform a more complex function.

* Organ systems are groups of organs that work together to carry out major bodily functions.

For example, your stomach is an organ made up of muscle tissue (for churning food), epithelial tissue (lining the stomach), connective tissue (holding everything together), and nervous tissue (controlling digestion).