Groups of tissues organized to perform some common function are called organs.

Here's a breakdown:

* Cells: The basic unit of life.

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

* Organs: Structures made of different tissues working together to perform a complex function (e.g., the heart, lungs, stomach).

* Organ Systems: Groups of organs working together to perform a major bodily function (e.g., the circulatory system, digestive system).

* Organism: A complete living being made up of all the organ systems working together.