A group of cells that perform a particular function is called a tissue.

Here's a breakdown:

* Cells: The basic unit of life.

* Tissue: A collection of similar cells that work together to perform a specific function.

* Organ: A structure made up of different tissues that work together to carry out a more complex function.

* Organ System: A group of organs that work together to perform a major bodily function.

For example, muscle tissue is made up of muscle cells that contract and relax, allowing for movement. Nervous tissue is made up of nerve cells that transmit electrical signals, allowing for communication throughout the body.