Cells: Basic unit of life.
Tissues: Groups of similar cells that perform similar functions. (EX: all nerve cells in the brain make a tissue.)
Organ: Groups of different tissues that work together to perform a specific function. (EX: The heart is made of nerve, blood, & muscle cells.)
Organ Systems: Groups of similar organs that work together to perform a specific function. (EX: Circulatory, nervous, & digestive systems.)
Organism: Can only live if all organ systems work together.