Whilst single-celled organisms perform functions adequately enough to live, they can't be good at all of them because they're just one cell! Each function requires a specific type of cell, e.g. sperm cells are designed to swim long distance to fertilize and muscle cells are long and stretchy so they can lengthen and contract. This is where CELL DIFFERENTIATION comes in!
Multi-cellular organisms have cells which are adapted to suit their function. Initially, they all start off as identical STEM CELLS. As the stem cells mature, each cell develops the charateristics required for it to perform its function well i.e. the cells become SPECIALIZED. It is this specialization that is known as CELL DIFFERENTIATION.
All the cells in an organism come from mitosis division of a fertilized egg. They all contain exactly the SAME GENES. "But then how is each cell different?!" you ask. Well....
Every cell contains the genes required for it to develop into any type of cell in the body. However, only a few of these genes are activated in each cell, so that you don't end up with an artery-muscly-eyeball-fingernail-spermy thing in your body!