Cells are the simplest building bocks of life. Humans consists of trillions of cells all working together.
Unicellular organisms consist of only one cell. This cell must be able to do all life functions. In muti-cellular organisms the cells can share their jobs - they can specialise on different functions.
Many cells with the same function form a tissue,
many tissues working together form a organ,
the system of all organs makes an organism.