Cells are referred to as the building blocks of an organism because they are the basic, structural, and functional unit of life, from which whole organisms develop.
Except for simple unicellular organisms, all existing cells of multicellular organisms stem from the multiplication of previously existing cells. This phenomenon (mitosis) illustrates how cells multiply to become multicellular organisms.
Some unicellular organisms (volvox and spirogyra) can exist in unspecialized colonies.