In an organism such as the human body, there are four different levels of organization: cells, tissues, organs, and organ systems:
Cells make up the most basic level of organization; a cell is the building block of a living organism.
Tissues are composed of groups of cells that work together and have a similar structure and function.
Organs are groups of tissues that work together to perform a certain function.
Organ systems are made up of groups of organs that work together in order to carry out a certain function.