The cell is the basic unit of life. Some organisms are made up of only one cell like E. Coli, while other organisms are made up of many cells working together, like humans.
Cells are capable of very diverse specialized functions.
In multicellular animals, cells do everything from absorb nutients to transmit electrical brain signals. Each cell is responsible for it own maintenance as well as any specialized functions it may preform. Cells may contain internal organelles which serve to compartmentalize cellular functions. In the figure below we see a cut away picture of an animal cell which contains many organelles. Each oragnelle has a specific function associated with it.