Just like us, cells grow old and die. When old cells die, new ones replace them. For example, a blood cell in our body lives for about 120 days. Another example is our skin cells. We shed our skin cells about every 35 days.
Here is a amazing fact for you, if your skin cells are replaced every 35 days, by the time you are 20 years old you would have replaced your skin cells roughly about 200 times.
http://askabiologist.asu.edu/content/cells-divide
BBC Secret Universe: The Hidden Life of the Cell: http://youtube.com/watch?v=4GZXRMG5i_w
What is a cell: http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html