Unicellular and multicellular organisms are similar in many ways, but the main difference comes in the form of their cellular makeup.
Unicellular organisms are made up of only one cell (the prefix "uni" means one). All life processes are carried out in one single cell! Unicellular organisms are often considered simple creatures and are far less complex than most multicellular organisms. Examples of unicellular organisms include bacteria (E. coli, Salmonella, etc.), yeast (Candida), and amoebas.
Multicellular organisms are made up of more than one cell (the prefix "multi" means many). Multicellular organisms are often composed of millions or trillions of cells (in the case of animals and humans) and are considered to be more advanced on an evolutionary scale.
It is an advantage for an organism to be multicellular rather than unicellular. Multicellular organisms have the ability to grow larger, to be more complex, and to differentiate cells. This means multicellular organisms have different types of cells used for specialized purposes (Ex: nerve cells, muscle cells, brain cells, etc.).
Even though unicellular organisms differ from multicellular organisms in terms of cellular makeup, they do have quite a few things in common. Both unicellular and multicellular organisms share the characteristics of life: they grow, respond to stimuli, maintain homeostasis (an internal balance), reproduce, pass on genetic material to offspring, and obtain or use energy.