Cells are the structural and functional units of all living organisms. Some organisms, such as bacteria, are unicellular, consisting of a single cell. Other organisms, such as humans, are multicellular, or have many cells—an estimated 100,000,000,000,000 cells! Each cell is an amazing world unto itself: it can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Even more amazing is that each cell stores its own set of instructions for carrying out each of these activities.
(taken from http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html)
Image taken from: http://www.mcb.ucdavis.edu/images/advising/cbi/cell-biology.jpg