The cell is the fundamental basic unit of life. A cell is made up of organelles which are the smallest biological level of organization in living organisms.
A group of cells joins together to form a tissue that performs a specific function. Examples of tissues in animals include nervous tissue, connective tissue, muscle tissue, and epithelial tissue.
A group of two or more tissues combines to form an organ. This means an organ function depends on the tissues making it. Examples of organs are lungs, heart, kidney, etc.
More than one organ combine to form an organ system such as the respiratory system, urinary system, digestive system, etc.